I wonder how long the facebook fad will last? I call this a fad only because weren't people saying about the same thing with myspace? I don't here anybody making many comments about them much.
Everything kinda runs hot and then cold, or Cold slow start then Hot. I feel the key is to reinvent your site every few years is important and by that I feel is more then just a theme change.
I feel google by slowing adding services has got that down, yet never lost track of how they started with just a search engine.