Giveaways & Facebook

If you are hosting a giveaway on your blog, can you require a contestant to "like" a company's page for entry into the contest?

The way I have always understood it, is that you cannot. But I have been seeing this requirement on a number of blogs as well as being requested by a number of companies. I have read over the guidelines, but those are all for hosting a giveaway on your actual Facebook page.

Does anyone have any advice or an opinion on this, or better yet the actual rules about this?

Thanks!