Here are some quick examples:
Did you add?
"This promotion is in no way sponsored, endorsed or administered by, or associated with, Facebook. You are providing your information to [disclose recipient(s) of information] and not to Facebook. The information you provide will only be used for [disclose any way that you plan to use the user's information]."
No? Then it's illegal.
Do you notify winners through Facebook, such as through Facebook messages, chat, or posts on profiles or Pages? Yes? Then it's illegal.
So how do you prevent this from happening? Easy, just follow the
Facebook Promotions Guidelines
PS- To report a Facebook page that is running an illegal promotion, just click on the 'Report Page' link on the bottom left of its page.