Do you think most users will care about this? They'll just want to know if they can still see what their friends are doing, who has posted new photos, etc.
What if the new owner decides to "generate revenue" by selling your personal information to the highest bidder, and then turns the site off? Reselling the information is cheaper than keeping a bunch of programmers and sysadmins employed, after all.
class action lawsuit perhaps?
You plan to ask for a refund? Nobody asked or paid 500 million users to put their entire lives online.
"But your honor, I didn't actually read the agreement I signed when I joined Facebook."
Yeah, that sounds pretty good, but I would have a backup strategy just in case...