The short answer: in the top-level directory of your web server.
The "/socialmedia.txt" file is a text file, with one or more records looking like this:
http://facebook.com/firechip
http://twitter.com/ajsb85
<link type="text/plain" rel="socialmedia" href="http://domain/socialmedia.txt" />
43 Things Goal setting and achievement
Academia.edu Social networking site for academics/researchers
About.me Social networking site
Advogato Free and open source software developers
aNobii Books
AsianAvenue A social network for the Asian American community
aSmallWorld European jet set and social elite world-wide
Athlinks Running, Swimming
Audimated.com Independent music
Bebo General
Biip.no Norwegian community
BlackPlanet Black Americans
Blauk Anyone who wants to tell something about a stranger or acquaintance.
Blogster Blogging community
Bolt.com General
Busuu Language learning community (headquartered in Madrid, Spain)
Buzznet Music and pop-culture
CafeMom Mothers
Care2 Green living and social activism
CaringBridge Not for profit providing free websites that connect family and friends during a serious health event, care and recovery.[34]
Classmates.com School, college, work and the military
Cloob General. Popular in Iran
CouchSurfing Worldwide network for making connections between travelers and the communities they visit.
CozyCot East Asian and Southeast Asian women
Cross.tv Faith Based social network for Christian believers from around the world
Crunchyroll Anime and forums.
Cyworld General. Popular in South Korea.
DailyBooth Photo-blogging site where users upload a photo every day
DailyStrength Medical & emotional support community - Physical health, Mental health, Support groups
delicious Social bookmarking allowing users to locate and save websites that match their own interests
deviantART Art community
Diaspora* Decentralized, privacy aware, general (open source)
didlr Drawing service
Disaboom People with disabilities (Amputee, cerebral palsy, MS, and other disabilities)
Dol2day Politic community, Social network, Internet radio (German-speaking countries)
DontStayIn Clubbing (primarily UK)
Draugiem.lv General (primarily LV, LT, HU)
douban Chinese Web 2.0 website providing user review and recommendation services for movies, books, and music.
DXY.cn Chinese online community for physicians, health care professionals, pharmacies and facilities
Elftown Community and wiki around Fantasy and sci-fi.
Elixio Business executives jet set and global elite.
English, baby! Students and teachers of English as a second language
Epernicus For research scientists
Eons.com For baby boomers and mature internet users age 40 and beyond.
eToro Social Investing, Finance
Experience Project Life experiences
Exploroo Travel Social Networking.
Facebook General. Photos, Videos, Blogs, Apps.
Faceparty General. Popular UK.
Faces.com Adult Social Network, Mainly UK & USA
Fetlife People who are into BDSM
FilmAffinity Movies and TV Series
Filmow Movies and TV Series
FledgeWing Entrepreneural community targeted towards worldwide university students
Flixster Movies
Flickr Photo sharing, commenting, photography related networking, worldwide
Focus.com Business to Business, worldwide
Formspring social Q&A website
Fotki Photo sharing, video hosting, photo contests, journals, forums, flexible privacy protection, friend's feed, audio comments and unlimited custom design integration.
Fotolog Photoblogging. Popular in South America and Spain
Foursquare Location based mobile social network
Friendica Distributed, federated, privacy aware, open source, general
Friends Reunited UK based. School, college, work, sport and streets
Friendster General. Popular in Southeast Asia. No longer popular in the western world
Frühstückstreff General
Fuelmyblog Blogging community
FullCircle Geosocial networking and location-based services portal for mobile devices
Gaia Online Anime and games. Popular in USA, Canada and Europe. Moderately popular around Asia.
GamerDNA Computer and video games
Gapyear.com Travel social network
Gather.com Article, picture, and video sharing, as well as group discussions
Gays.com Social network for LGBT community, Guide for LGBT bars, restaurants, clubs, shopping
Geni.com Families, genealogy
GetGlue Social network for entertainment
Gogoyoko Fair play in Music - Social networking site for musicians and music lovers
Goodreads Library cataloging, book lovers
Goodwizz Social network with matchmaking and personality games to find new contacts. Global, based in France.
Google+ General
GovLoop For people in and around government
Grono.net Poland
Habbo General for teens. Over 31 communities worldwide. Chat room and user profiles.
hi5 General. Popular in Nepal, Mongolia, Thailand, Romania, Jamaica, Central Africa, Portugal and Latin America. Not very popular in the USA.
Hospitality Club Hospitality
Hotlist Geo-Social Aggregator rooted in the concept of knowing where users' friends are, were, and will be.
HR.com Social networking site for Human Resources professionals
Hub Culture Global influencers focused on worth creation
Hyves General, mostly popular in the Netherlands.
Ibibo Talent based social networking site that allows to promote one's self and also discover new talent. Most popular in India.
Identi.ca Twitter-like service popular with hackers and software freedom advocates.
Indaba Music Online collaboration for musicians, remix contests, and networking.
Instagram A photo and video sharing site
IRC-Galleria Finland
italki.com Language learning social network. 100+ languages.
Itsmy Mobile community worldwide, blogging, friends, personal TV-shows
iWiW Hungary
Jaiku General. Microblogging. Owned by Google
Jiepang Location based mobile social network. In Chinese language
Kaixin001 General. In Simplified Chinese; caters for mainland China users
Kiwibox General.
Lafango Talent-Focused media sharing site
LaiBhaari Marathi social networking
Last.fm Music
LibraryThing Book lovers
Lifeknot Shared interests, hobbies
LinkedIn Business and professional networking
LinkExpats Social networking website for expatriates. 100+ countries.
Listography Lists. Autobiography
LiveJournal Blogging. Popular in Russia and among the Russian-speaking diaspora abroad.
Livemocha Online language learning
Makeoutclub General
MEETin General
Meetup (website) General. Used to plan offline meetings for people interested in various activities
Meettheboss Business and Finance community, worldwide.
MillatFacebook General, Created in Response to Facebook [193]
mixi Japan
MocoSpace mobile community, worldwide
MOG Music
MouthShut.com Social Network, social media, consumer reviews
Mubi Auteur cinema
MyHeritage family-oriented social network service
MyLife Locating friends and family, keeping in touch (formerly Reunion.com)
My Opera Blogging, mobile blogging, photo sharing, connecting with friends, Opera Link and Opera Unite. Global
Myspace General
Nasza-klasa.pl School, college and friends. Popular in Poland
Netlog General. Popular in Europe, Turkey, the Arab World and Canada's Québec province. Formerly known as Facebox and Redbox.[213]
Nexopia Canada
NGO Post Non-Profit news sharing and networking, mainly in India
Ning Users create their own social websites and social networks
Odnoklassniki Connect with old classmates. Popular in Russia and former Soviet republics
Open Diary First online blogging community, founded in 1998
Orkut General. Owned by Google Inc. Popular in India and Brazil.[226]
OUTeverywhere Gay/LGBTQ Community
PatientsLikeMe Online community for patients with life-changing illnesses to find other patients like them, share their data with others, and learn more about their condition to improve their outcome.
Partyflock Dutch virtual community for people interested in house music and other electronic dance music
Pingsta Collaborative platform for the world's Internetwork Experts
Pinterest Online pinboard for organizing and sharing things you love
Plaxo Aggregator
Playfire Computer and video games
Playlist.com General, Music
Plurk Micro-blogging, RSS, updates. Very popular in Taiwan
Poolwo Social networking site from India
Qapacity A business-oriented social networking site and a business directory
Quechup General, friendship, dating
Qzone General. In Simplified Chinese; caters for mainland China users
Raptr Video games
Ravelry Knitting and crochet
Renren Significant site in China. Was known as 校内 (Xiaonei) until August 2009.
ReverbNation.com Social network for musician and bands
Ryze Business
ScienceStage Science-oriented multimedia platform and network for scientists
Sgrouples General, with focus on privacy rights
ShareTheMusic Music Community. Sharing and listening to music for free and legally
Shelfari Books
Sina Weibo Social microblogging site in Mainland China.
Skoob Collaborative social network for Brazilian readers
Skyrock Social Network in French-speaking world
SocialVibe Social Network for Charity
Sonico.com General. Popular in Latin America and Spanish and Portuguese speaking regions.
SoundCloud Repository of original music pieces and networking.
Spaces Russian Social Network targeted to mobile phone users
Stage 32 US-based social network and educational site for creative professionals in film, television and theater
Stickam Live video streaming and chat.
StudiVZ University students, mostly in the German-speaking countries. School students and those out of education sign up via its partner sites schülerVZ and meinVZ.
Students Circle Network A Social Network connecting students, teachers and institutions to course resources, study groups and learning spaces.
StumbleUpon Stumble through websites that match users' selected interests
Tagged General.
Talkbiznow Business networking
Taltopia Online artistic community
Taringa! General (primarily Argentina)
TeachStreet Education / Learning / Teaching - More than 400 subjects
TermWiki Learning / Languages / Translation - 1.2m terms in more than 1300 subjects
The Sphere A Private Online Social Luxury Network with Exclusive Personalized Services
TravBuddy.com Travel
Travellerspoint Travel
tribe.net General
Trombi.com French subsidiary of Classmates.com
Tuenti Spanish-based university and High School social network.
Tumblr Microblogging platform and Social Networking Website.
Twitter General. Micro-blogging, RSS, updates
Tylted Mobile social game network[303]
Vkontakte General, including music upload, listening and search. Popular in Russia and former Soviet republics.
Vampirefreaks.com Gothic and industrial subculture
Viadeo Global Social Networking and Campus Networking available in English, French, German, Spanish, Italian and Portuguese
Virb Social network that focuses heavily on artists, including musicians and photographers
Vox Blogging
Wattpad For readers and authors to interact & e-book sharing
WAYN Travel and lifestyle
WeeWorld Teenagers - 9 to 17
We Heart It Image-based social network focused on inspiration, expression and creativity
Wellwer Community without borders, where sharing is everything.
WeOurFamily General with emphasis on privacy and security
Wepolls.com Social polling network
Wer-kennt-wen General
weRead Books
Wiser.org Online community space for the social justice and environmental movement[326]
Wooxie Blogging and micro-blogging
WriteAPrisoner.com Site networking inmates, friends, family
Xanga Blogs and "metro" areas
XING Business (primarily Europe (Germany, Austria, Switzerland))
Xt3 Catholic social networking, created for World Youth Day 2008
Yammer Social networking for office colleagues
Yelp, Inc. Local Business Review and Talk
Zoo.gr Greek Web Meeting point
Zooppa Online Community for Creative Talent (host of brand sponsored advertising contests)