You are worth as much as the problem you solve for the buying party.

Facebook is (rightly or wrongly) perceived by the market as:

1) Losing its cool

2) Suffering from user flight

3) Still not offering significant competition to short-message social networking

For a $116B company, offering 2.8% of its total value to fight off what might be a a future 20%+ loss in value is a no brainer.