This article is interesting, the guys at 37signals are about as practical as you can get. Take a read of this. When they were valued at 30 billion they hadn't figured out how to substantially bank on users and given circumstances unlikely they ever will.
Zygna really figured it out. Phew!
David Heinemeier Hansson (creator of Ruby on Rails (37signals basecamp) on his thoughts on Facebook being irrationally valued.