WARNING WARNING WARNING
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This plugin doesn't work anymore, because of facebook api changes. Sorry about that...

Usage:
1- include jquery autocomplete plugin (this one: http://bassistance.de/jquery-plugins/jquery-plugin-autocomplete/)
2- include that file
3- add the following javascript code:

<script type="text/javascript">

    $().ready(function() {

	FB.ensureInit( function() {
		//input with id = selectorField needs to be input type = text
		$("#selectorField").friendselector({hiddenfieldname: "' . $name . '"});
	 });
    });

</script>

4- include the field:
<input type="text" id="selectorField" style="display:none;" class="fbs_single" />


It adds ul/li and hidden fields with attribute name=fbfriendsfields (the id of the selector field) but you can customize the "dropdown" option by redefining jQuery.fn.friendselector.formatOption

Options:
- hiddenfieldname: the name of the selected friend
- includecurrentuser: defines is the current logged users should be part of the options

If the input field has a cssclass "fbs_single", then you can only select one friend (the selector field is hidden).
If it has not a css class "fbs_single", it will allow you to select as many friends as you want.

Symfony Widget:
I also created a Symfony widjet that uses this jquery plugin. Just copy the widget folder into your lib folder. a Symfony plugin is on its way... 

Limitations/Disclaimer:
Friends that have been already selected can be selected as many time (is not in "fbs_single" mode).
It is javascript based! The friends' list is cached (thanks to the autocompleter plugin) once per page.
I didn't try it with more than 200 friends...