[rogerbinns]

Note that you don't really need to confirm everyone who participates, only people who win. You should make it clear that should someone win you will go full forensic on them. Providing you've saved enough data you can go back and correlate their activity with similar activity to see if cheating appears to happened. You can do investigation with humans which makes attacks harder, and also scales (you only need to investigate the limited number of winners rather than all participants).

As 0xEA mentioned you can go network effect on them too. For example you can require that winners had Facebooked/Tweeted something at the time of their entry. When someone wins you can go back an check that which means that someone cheating would have to do so with lots of dummy accounts. And of course the messages form advertising for your contests.

You can also make them nominate FB/Twitter friends to share their prizes should they win at entry time. Again this makes life a lot harder to cheat as it gives your win investigations more to work with.