Apart from this place, only Facebook. Which I have a love/hate relationship with. I'd be lonelier without it, but also less stressed. As I'm so crap at communicating with friends, it's good to have Facebook as a low-pressure way to do it. But when I'm depressed or anxious, I don't really interact with people, just read lots of stuff in the news feed that makes me worse. Wierdly enough, a lot of my best conversations on Facebook are with people I've barely met in real life. But I never feel I can be 100% honest on there. Probably just as well, given their privacy issues.
I use Soundcloud, but don't make much use of its (limited) social network functions- I mainly post links on Facebook, or swap links via email with a musician friend.
I tried Twitter, and still have an account. I don't get how to use it, though. I couldn't seem to follow a conversation or filter anything interesting out of the flow. I think it's optimised for someone who has a smartphone in their hands 24/7, not someone like me who's only online a few hours a week. The one thing I liked was the character limit, because I like writing and it's an interesting challenge. My few tweets were mostly haiku. But most of the tweets I saw were built around pictures, links or videos, which felt like cheating to me!
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You're so vain
I bet you think this sig is about you