M.L.
Just so you know, running contests on facebook is against the rules and you can actually lose your facebook business page. Of course, it is not very regulated.
What is allowed, if you have another website/blog that you run the contest on, then you can announce it on your facebook page.
Contests that usually work to generate other people to your site, aer ones where the people applying to the contest has to also share. For instance, they have to like your facebook page, then also promote the link on their facebook/twitter/blog as well.
Ideas the people could share for entry could be, worst Christmas present received, biggest party disaster story...