The idea is simple: All you do is post something, and in the post you tell your fans that for the first people who share it, they get some kind of related goodie. On Twitter for example, you could say "The first 50 people to retweet this get _____ sent to their inbox."
It might be more of a challenge to do it on Facebook, since pages can't send messages to people who liked the page, but there must be a way to. As far as Twitter, all you'd have to do then is look at how many people retweeted and then send them a link to download the goodie.
I haven't had a chance to try this yet, but it seems like you could get people to virally fan your profiles pretty fast. After all, why send out tons of posts hoping someone will see them when you can put a little more effort into one that gets you 10x or 20x the amount of exposure?
Now just imagine how fast people would like your pages if you did this five times a day.