Size doesn't matter. Utility does. If Facebook were to start completely over again against the current competition, I think they would still rise to prominence

Size doesn't matter. Utility does. If Facebook were to start completely over again against the current competition, I think they would still rise to prominence without much trouble. What makes Facebook different, in short, is that while other services offer a place to talk, Facebook is built as a utility in a way other sites aren't. It's the new big communication medium. The people that use it are using it as a replacement to email and IM. That's a huge deal, because it suggests Facebook has the potential to last as long as email has. Your suggestion that people abandon things all the time is countered by AIM, which has been the prominent IM service for a decade.