There's also your attention surface. I typically read threads/posts from a variety of communities, but might prefer to reply only on some, and lurk on the others. I'm sure this is true for a lot of people, from anecdotes I've heard.
Yep. If it's about programming, game design, board games, video games, writing, I'll feel confident I have something to say and/or want to contribute. But I also read discussion about music composition, hiking, art, diy, history, philosophy, etc, and I would almost never post in those subjects (at least not at this point in time), as those aren't my focus, just other subjects I'm curious about.
