Couldn't your Point #1 also be said of MySpace before?

The problem for Facebook is that the cost for the users of switching to a newer social network site is practically zero. A little bit of time to create your profile, but no real significant costs.

If 3 of your most significant friends from FB emailed you about a new, free, social site, would you ignore their emails, or would you go check it out and maybe join them? If you joined them, how many people would YOU attempt to drag over?