Youtube I guess, though that's pretty much where I go to make half arsed arguments about things I don't actually believe. Legitimately trying to argue on Youtube is like winning a free lobotomy.
Facebook sort of. I don't post status updates and I have chat turned off so I don't actually have to talk with anybody. If anybody wants me they just send me a message assuming I'm offline and I respond if I want to (my best friends being wise to the fact I do this). I essentially only use it to communicate with people for "hey, you out later?" or something.
IRL interaction > bad grammar via Facebook.
Anything else I don't really regularly use enough to mention. I just browse places.
tumblr
if Skype counts, Skype
Facebook
last.fm(rare, don't really talk to anyone there right now)
if Flight Rising counts, there's also that. but I'm not too active there right now.
there's been others, but I've kinda ditched all of them. I have a YouTube account, but I'm not any kinda active commenter.
Youtube for videos
4chan/reddit (rarely but from time to time)
tumblr
tbt/ATRL (sometimes I get social on forums, mostly ATRL cuz I can cuss and get away with more and its about music hihi)
Lastfm (just 2 chart music, stopped using it socially around 2011)
twitter/fb (have accounts but never post?!??!)
Tumblr and facebook, mainly. One of my favorite websites went down and they set up a page on facebook for the remaining users. And I live far away from my hometown, so it's nice to stay in contact with old friends. Tumblr for random things, really.
I live on Twitter. I used to live on Tumblr but have abandoned it in the past 6 months... I'm kinda over it and any good friends I made on there I have on Facebook now anyway. Facebook sparingly, though more often lately.