What are the most effective ways to monetize a Questions and Answer or Communities web sites?

Facebook sells your data to advertisers. The more members, the more data, the more revenue. They have no shortage of inflow.

Fluther is funded by renting out advertising space. Users on this site don’t see any ads, but people who are not logged in do see ads.

Facebook is funded by advertising and has a ton of personal data that can be used.

Ask MetaFilter charges $5 to create an account.

If you’re not paying for it, you’re not the customer. You’re the product. So when you talk about these services being free, don’t forget that the money is made behind the scenes. The price we pay is invisible to us, but valuable to someone else whose face we never see. It’s interesting you should ask this question on Fluther of all places, though, seeing as the site has had some major cash flow problems lately. If we had answers to your question, we’d still have a full time Community Manager.