how to get free facebook likes?

[me]

That depends on the kind of likes you want. If you want likes for your page, you can easily get some using youlikehits.com where you can either buy likes (not recommended), or you can get points in exchange for other liking, video views, viewing websites for like 20 sec, etc...
It's a good way to get likes and views on almost anything: Facebook, Twitter, YouTube, Pinterest... plus the fact that it's a sort of exchage, so you get real people liking your page and not fake accounts as those who simply sell like 50k for $50 and other scams.
But it you need likes on a certain post, then there is no option I can think of other than good marketing