Well, Google has essentially the same number of users and is traded at 190B capitalization. So, a Google user is worth over $300. As people spend more time on FB then at Google, I can understand why FB user is seen as $100 asset.
http://en.wikipedia.org/wiki/History_of_Google#Financing_and...
http://blogs.wsj.com/digits/2010/03/04/investors-bet-on-pric...
Goldman invested into FB - so they can now act as marketmaker for their customers.
Virtual gifts aren't changing the world anytime soon.