Youtube isn't hard, it can be done via good old torrenting - you already can stream videos on some sites this way. Of course it won't be 100% like Youtube, since you can't take down your videos once they are there, but we can just call it a feature.

Facebook, however, is much different - it is inherently stateful, and immutability is unwanted here - imagine you want to delete some photos that you have made while you were drunk, or a post that you now regret - blockchain would make sure that it stays there forever.