Joanna Glasner at Wired has written the article Are Socialites Still Networking? that takes a look at whether social networking sites are living up to the hype.
She covers the split into two sub-groups: meeting friends and professional networking.
Personally I think the former is mostly useless, but find the latter of some use via systems such as Linked In that offer limited access (on a recommendation basis) to people that may have significant value for you to get hold of, but which you might have no idea of how to reach.