Social Networking is a term used to describe websites where you network socially. It's a total waste of time.
Uh, your family can stalk you?
One advantage is that on some sites you can program your web page to play My Humps when anyone visits it, which everyone really appreciates.
You end up "connecting" with people who you never cared about, and who never have anything interesting to say. At nice places that aren't social networks (like oh for example, spacefem) everyone is required to have something interesting to say, otherwise we totally won't care about them.
You're supposed to use your real name, which makes everyone less interesting. You can be your real self in real life, why would you want to do it on the internet?
You get in trouble for posting pr0n.
Your family is now online. Not your cool us family, your FAMILY... aunts, uncles, cousins, parents, in-laws, out-laws, brothers, sisters, sisters' pets.