I joined Facebook back in June. I did it because the owner of the Post Secret site started a page there and I wanted access to it. I put up a bare-bones profile, did a couple of searches and "friended" a couple of people. Then I pretty much left it alone on the theory that this was largely a site for the under-30 crowd. Until about five weeks ago. Then, all of a sudden I started getting messages and friend requests as a bunch of my old friends from high school and college - and I mean at least a dozen people, all over 45 - joined the site and started searching for people. I've reconnected with a number of them now. With some I've exchanged just a message or two, with others I feel like I've really resumed a long-dormant friendship. It couldn't have come at a better time.
As a side benefit, Facebook is now allowing people to set up pages for their businesses. I've done one for the inn. I'm not sure if I'll see a benefit from it, but at the moment it's free and it can't hurt. It may help me draw in a younger generation of B&B goers.
I also recently joined Linked In. That site is more for business networking. There is some overlap between the friends I've found there and the ones on Facebook, but I've found a number of people on Linked In who have probably never considered joining Facebook.
If you've been getting invitations to join either of these sites, take a chance and do it. You might be surprised at who you find there. Be warned, though, both can be a bit of a time-suck if you let them.