Out-law reports:
“Facebook considers itself worth around a quarter of the $15 billion valuation on which a Microsoft investment in the company was based. The company considered itself to be worth $3.7bn in the middle of last year, according to court documents.”
The valuation emerges from court documents in the case brought by former university friends of Facebook founder Mark Zuckerberg who claim the idea for Facebook was partly theirs.
Mark Zuckerberg still has lots of friends on Facebook.
www.out-law.com
Understanding social networks: a good place to start would be this Wikipedia page.