Since Facebook follows the popular "Double Irish Arrangement:"

http://en.wikipedia.org/wiki/Double_Irish_arrangement

The location of its legal tax headquarters is up for debate; Additionally, Facebook has corporate offices all over the world in different countries: https://www.facebook.com/careers/locations Italy, India, Canada, New Zealand, Belgium, etc.

Clearly it's not just a "US" company and only has to follow "US" laws.