I'm in the very beginning stages of searching out Mandarin-speaking communities online, so I don't know of any groups yet. But I'm interested in hearing what other people have to say.
To be honest, I had kind of written WeChat off as a viable community for me, since only about 5 people in my current social network use it, and it doesn't seem to be a great tool for meeting new people (at least from where I am in LA). I just downloaded Weibo onto my phone this morning, which does seem to be better in this regard.