According to this New York Times article, Facebook played an important role in the recent events in Egypt that brought down their government. Indeed, American politicians are urging Facebook to go further in aiding such uprisings:
Last week, Senator Richard J. Durbin, Democrat of Illinois, urged Facebook to take “immediate and tangible steps” to help protect democracy and human rights activists who use its services, including addressing concerns about not being able to use pseudonyms.
Of course, I’m pretty certain that Facebook would be expected to immediately share any and all info about its users should the U.S. government demand it (regardless of any Constitutional prohibitions) as they have been known to do in the past.
It should also be noted that, while Facebook has no policy specifically prohibiting the use of its network to bring down a government, you’d better not be posting pictures of women breast feeding their babies. I mean, there are limits to what a socially responsible mega-network will do in the name of freedom…