Choose a random user UI.

I dont think we have this concept at facebook but on the russian net it is pretty popular tactic to gain social traffic.
To be short, everyone have to make make a repost of a certain contest post, and in the end we use a site like what I made to choose a random user who gets a prize.
On my landing page we enter a link to the post, number of winner and the rest is done by the site =)