Actually, Facebook and YouTube might be able to dodge a bullet here. Facebook is about to open (or they might already have opened, I haven't checked lately) a server farm in Canada's Far North. Since in order to do so they need to legally purchase the land, and in order to do THAT they have to be an official separate entity in Canada (basically meaning they have to exist as a standalone company here), all they would have to do is shift the most important part of their service to the new server farm, officially move their head offices to Canada, and then begin server farm expansion (or just make more in other international locations). Service would be intermittent for the first few months, but it wouldn't die.
YouTube is owned by Google, as I'm sure you're aware. It would simply cease to exist inside of the US, and since I know for a fact Google operates outside of the USA, it would be a simple transfer of service to servers outside of the States in order to continue operating.
Sucks to be American right now, I gotta say. You'll be losing access to so much.....I predict that Mexican and Canadian immigration levels will skyrocket.