[hippich]

(links below are NSFW)

I tried this experiment in the summer of last year. Did not work out well in my case, but may be someone will use this idea to execute better.

It starts with this - http://orgasms.org/promotion where to enter a promotion, you need to enter email address. Once u entered, you get in a queue like this - http://orgasms.org/promotion-status?email=johndoe%40example.... which tells you that you are Nth in the list of applied and current winning entry is 10th. So if you just applied, for you to get closer to 10th position you need to share it with you friends so they get applied too :)

There were no anti-fraud anything implemented, since it was just an experiment, but in real-world situation comprehensive IP/Cookies/etc detection of same user submission will be required obviously.