I participate a
lot, but almost exclusively in the (HTML5) Canvas tag. Because I don't answer much else I've got a smallish reputation for a frequent visitor(12K).
I'm the top answer-er for the Canvas tag, having answered around ~10% of all questions ever asked about Canvas on StackOverflow.
I would say that reputation numbers don't necessarily measure engagement per se in S.O. very well.
I've found that if I answer a general JavaScript question, even if its an absurdly simple one, I garner more reputation in a few minutes than if I I give a detailed answer to 2-3 in depth canvas questions.
If I wanted to pad my reputation I could definitely answer more general JS questions that crop up, but I really enjoy helping people with their canvas projects, so I sorta stick to that domain and try to help with my slightly-more-unique knowledge.
Anyway, reputation numbers aren't that important to me. At the end of the day, this is the only encouragement I need:
http://i.imgur.com/POZmt.png