It's still easy to game, because there's no "dislike" button. Show 50% of your visitors a really valuable awesome page, and the other 50% the viagra spam. You'll get a ton of "likes" from the awesome page, and hence a lot of traffic for the spam.
What if you add in whether you're friends with anyone who liked it. Or at least a few degrees away.
Facebook already finds and deletes fake accounts. Trust me -- I've had my test accts blown away regularly!