We'll see how my s.o. reacts to it, but on my machine it does absolutely nothing.
In case you're wondering what is in my hosts file:
edit: regarding my s.o. it's been an interesting morning, this app seems to have opened her eyes to facebook in a different way. No more apps.
So I thought to myself 'ok, this once' knowing I'd be sticking my head out. It took me a minute or two to realize the cause and I thought that was a nice side-effect of solving this at the DNS layer, even when you're momentarily stupid this will still create enough of a barrier that you'll stop to think a bit longer.
I didn't know about the ghostery extension, so thanks to the person that posted that, that's another good solution.
FB has had its use for me (it found a bunch of long lost people), I haven't been back since.
Other than the usual suspects and FB my host file is empty.
I can see how a huge hostfile would impact performance, and I would advice against using it to block a very large number of hosts.
Facebook uses a lot of subdomains like static.ak.fbcdn.net and there's no way you can include them all in your list.