I've picked up a couple of consulting gigs through providing decent answers im the Answers section.

If I have a few moments spare at the end of the day, I'll look through open questions and answer them. Sometimes they are interesting questions that I would like to think through myself; sometimes they land directly in my area of expertise. But it's a win-win you help someone, and at the same time advertise that you are a reasonable, helpful guy who knows at least something about an area.

By comparison, I find Facebook completely pointless.