Facebook would have been nothing if it weren't for the popularity of Myspace. Facebook also would have been nothing had Myspace continued to innovate and grow, giving users fewer compelling reasons to switch.
I'm not sure whether innovation and growth is the key to success in this field. Looking at the trends it seems like there's a natural saturation point where you reach max penetration, followed by an exodus. I really doubt it's possible to keep the ceiling going.
I don't think you can compare MySpace and Facebook. MySpace never really innovated. They never had a platform or began to do what Facebook did. They got bought out and lobotomized.
You're right. I posted it because the Myspace<->FB comparison is often used and I wanted to illustrate the trends for something that grows/saturates/declines.
trend for myspace: http://www.google.com/trends/?q=myspace
trend for facebook: http://www.google.com/trends/?q=facebook