I recently signed up for LinkedIn, which is a type of professional facebook. One interesting thing is that it is a subscription based model. There is advertising, and there are probably people trying to sell their wares on the forums or direct to people. But the cost of the subscription is not small. $20 per month.... I can get hardcore pornography for less than that. Extra things you get if you pay are things like being able to save profiles, view extended information. Which should realistically be included if they don't want a similar site to start up and taking all their users (say even facebook itself).
I have signed up to a few CPA/accounting forums and I even scored a contact which makes me feel less of a loner.