There's some deep part of your brain that instinctively wants to make connections with other human beings. Even when you do something as superficial as click a button on a website to confirm that somebody you've already known for ten years is your friend, that bit of your brain experiences a little 'ping' of happiness.
This is why social networks like Friendster, MySpace and Facebook work. Once they've tricked you into signing up, you have to find everyone else you know who is also on the service and connect to them. When you run out of people to connect to, you have to tell everyone you know to get an account so you can connect to them again. Because it's fun. And it's fun because people are fun, even if they're people you see every day, and have no need to interact with over the Internet.
Eventually you run out of real friends to add. At this point you have three options:
1. Find some other feature of the site to keep you logging in
2. Gradually lose interest, returning every month or two to see if anyone else has added you
3. Radically lower your standards
-- Charles Miller, "On Social Networks" (Sept 14, 2007)