If you state "I voted" on Facebook, your circle of friends (which is likely to include some people who are not likely to vote) will see that you voted and depending on what they actually think of you (extremes here are a good thing) they may be goaded into going out to vote.
NBC News article with all the links
Here's a link to a site with images you can use to change your profile pic: http://www.facebook.com/...
Happy voting!
p.s. Netroots Radio and Daily Kos Radio will be providing live radio coverage of the election results beginning at 6pm eastern tonight. Hope you'll tune in. You can listen to us via the Netroots Radio live stream on iTunes, on the mobile app 'Stitcher' or at our blog (linked above).