I just use Facebook to keep up with friends and as for procrastination. It's helpful for events, otherwise I just use it to procrastinate it and refresh mindlessly.

Twitter ends up being a dumping ground for random thoughts, or sharing things about life I find amusing.

I use Tumblr as an archive for whatever I find interesting, collecting interesting pieces of art, incoherent rants, and other random introspective ramblings. I find that a lot of my friends are more vulnerable on Tumblr than in real life.