I joined Facebook recently, just to find out what it was all about, since libraries are now trying to use it (rather unsuccessfully, as far as I can tell) to create 'communities' of their users.
A number of friends and acquaintances seemed to join up at about the same time, so I found a group almost immediately. However, I also found that the amount of use I made of the system was minimal and that it added little more to the experience of communicating than does my ordinary use of e-mail. I also found messages telling me that person X wanted to be my 'friend' - these turned out to be spammers, peddling pornography in general and porongraphic images in particular.
I don't really need any more spam - and am thankful that Gmail's system gets rid of most of it for me, so I have deactivated my account and do not plan to reactivate it at any time.
I suppose these social networking sites are of some use, but I can't really imagine what it might be!