Facebook all the way, all trough twitter is being used by celebs and important people in the society!
Other than, the ones to most likely use Tumblr a lot are bloggers...
I think you can't compare them. They all have different purposes.
Facebook is for connecting with friends or people you actually know
Twitter is for connecting with new people or people you may not actually know
Tumblr can be used to share things you find cool or interesting or just generally funny.
Facebook In my opinion, is for talking and connecting to people you already know.
Twitter is for meeting people you don't know with similar interests, and also tweeting those you do know, although it isn't practical.
Tumblr is for people who hold no real purpose.
They're used for different purposes. Facebook is used for catching up with friends, live chat, checking to see what people are up-to and to see the latest updates from each pages.
Twitter is for the importent people in society so people can see what there doing. Whether it's making a new film or going to the shops, people want to know about it.
Tumblr is for bloggers and people who want people to know about what they like/do with there sad and lonely life.