I could be wrong, but I'm fairly sure some years ago FB had a rule that you weren't allowed to force people to like your page to enter a competition. Last time I had to set up a competition for a client (a couple of years ago) that rule didn't seem to exist so either the rule has changed, or I dreamed it.

Either way I'm glad I have nothing to do with that sort of work anymore. Having quite a few friends in agencies I see the X likes Y posts all the time because they have to like a load of brands for their jobs. What a miserable waste of time.