Facebook, MySpace and their ilk are websites where you can put up your profile, details, blog and invite friends. These days with the denizens of the world in greater transience, people keep moving all over the place. These are useful tools to keep in touch, promote your interests etc.
MySpace is a very configurable platform - you can turn your web page into almost any form. There's been a flurry of activity recently from Facebook too - to make it's web page more malleable by releasing the Facebook Platform. However, there continues to be a difference in the culture of these two online communities. See below:
Video from CNET
Note-starts with an advertisement
Cartoon from BLaugh
At this stage MySpace is the leader by far, but this market can swing unpredictably.
Other interesting links
Feature Comparison between MySpace and Facebook
Tipping Point for Facebook
Related BBC Article, passed on to me by a colleague