Being a comic-book villain is quite a stressful job, it takes time, planning, and lets of man hours. That's not to say they don't have down-time, and just like the millions and millions of people out there who have Facebook and Twitter, why wouldn't they?
After all, supervillains are known for their pointless drawn out monologues, and what better way to monologue then Twitter.
*NOTE: This list was not put together by me, it's courtesy of the folks over at ToplessRobot*