Definitely Facebook. I have my Farmville, Fronteriville, CityVille...you get the picture. I will say I found the coolest thing Twitter the other night though. www.paper.li
Turns your tweets into a "newspaper" format. So much easier for me to read and to navigate.
I have to agree there are many fine apps out there that keep you from having to spend your time on Twitter. This can help for people who tend to use it for a time-sink.