I can't see the point in Twitter (neither could a number of my colleagues when it was mentioned at a dinner a couple of weeks ago, none of them used it).
MySpace was a genuine competitor, but allowing teenagers to style their own HTML made for some horrendous pages.
Facebook had a nice clean interface, as well as network effects. I was signed up to a similar service at the time - HighFive I think it was called, but the network effect was what snowballed with Facebook and led to its success. At the time I didn't see either one being better or more stable than the other.