Call me cruel, but I hate people who constantly post about how depressed and sad they are on social media like acting all woe is me.
That does seem heartless. Why do you feel that way about them being so depressed on social media? Are you just fed up with it all the self centeredness?
Yes, and I never send them public messages encouraging them to feel better.
There are a lot of attention seekers on social media, that is for sure but there are people who genuinely need help. It gets to be too much all the time, and is just not fun. Being more supportive to them might help them function better though, so try to be a better friend and see how that works for you.
No, I don't want to reward them for being so desperate.
I understand that you're not trying to encourage their attention-seeking behavior, but I'm sure many of them are really feeling terrible and would be appreciative of a kind word from you.