Carina of Carina’s Craftblog has a lot to say about bloggers who use Facebook as a vehicle for contests, votes, and giveaways. Among other things, she points out that using features such as the “Like” button in this way violates Facebook’s terms of use. Definitely something to think about. Go to her post.
I don’t know anything about Facebook’s terms of use but an author I “like” on there recently posted that she can no longer give away books on Facebook. So maybe they’re cracking down on it now.