Obviously, nobody is counting on making a living from a facebook page. but I'd put it in the same group as having a blog, twitter or youtube channel. Of course you're not going to search youtube for a photographer to hire, but people like that kind of things.
Specially if you're not marketing yourself in the top niche. For example, I work a lot with demo bands (Ones that don't have a contract yet, I'm not sure about the terminology), and my competitive edge is that they get a fair price considering that I can shoot them in less time, with less equipment and deliver less pics in the end than I would for a full blown band. On the other side, they don't have the budget to cover a 'real' shoot, so they wouldn't hire a photog at all with the prices the inflexible competition is quoting. And these guys can't get to know you unless you're somehow present on the web. So of course I'm not counting for clients who go to facebook and search "photographer", but the people who'll be looking for a photog and remember that they once saw something great on there.