Nice idea, maybe we could make a list of more domains to block:

Your ideas: * facbook.com * twitter.com * youtube.com

Mine: * stackexchange.com (yes!) * amazon.com (am I the only one using this so often?) * nytimes.com

It really seems like a nice idea to see how much we got used to these things when only a few years ago we didn't even knew we might need it :D