Or did you forget that Most of the users came from myspace because Facebook was better?
Hmmm, not necessarily, dude. In my country, for example, myspace was NEVER all that popular. In fact, most of my friends wouldn't even know what myspace is (other than "some social site that was never as useful or organized as FB).
Before facebook became popular, me and my friends used instant messengers (mostly ICQ or MSN) to communicate. Now those are just "outdated technology" nobody uses anymore. Hell, you always had the inconvenience of installing them on your system and having to update them. Since facebook is web-based, you don't need any of that.
The only thing FB is still missing now is video-chat. (or am I wrong?) Once FB has video-chat it would crush Skype, hammering the final pin in the IM grave.