"Worth frequenting...": sites like M.SE and MO are real gems, and I won't go into details, as I don't need to preach to the choir. Both these sites however are very focused upon the question/answer format making it unsuitable sometimes for discussion (e.g. discussing research papers, research avenues, discussion based soft questions) or organizing community based activities (e.g. finding people who might be interested in working on a research project with you).
Are there any communities that you frequent apart from M.SE or MO that fill this niche for you?
Please exclude from your suggestions the following (unless you have very good reason):
reddit.com's mathematics subreddits are a bit tricky, and I can't quite put my finger on "why". I suppose they're too far off into the "water cooler conversation" end, and worse, users are rewarded based on the whims of popular social dynamics, rather than production of useful content.
Blogs of some research mathematicians (Gowers, Tao, etc.), but they are not equipped to be proper communities, despite having pseudo-communities formed around them because of how highly frequented they are.
Quora: good luck not getting drowned out there.