Honestly? I use Twitter most often because its really easy to just glance at briefly. Facebook Pinterest Tumblr and DeviantArt all suck me in for hours at a time O.o So I check my Twitter most frequently ^-^
I still use facebook the most because all my friends use it and I don't talk to some of them at all except for through facebook, but I'm getting increasingly annoyed with it. I am getting to like tumblr more and more, but I wish all my friends would move over to Google+ so I can communicate with them over there instead.