The most powerful online tool for any social network, which wants to display ads is local data. Facebook allows users to check-in to different popular locations around the world, precisely to tap into such data. Usually it helps the local brand of a popular location in a city when people check-in while visiting it. It sort of ends up being a word of mouth promotion for a restaurant, hotel or even a large shopping centre.
Here is an infographic of the most social cities released by Facebook along with their most checked-in landmarks. 
(click on image to enlarge)
Look at the infographic above it shows that the most popular landmarks across the world where people check-in are sports stadiums.
I still think of this feature on Facebook as highly under utilized, especially compared to something like Google+ local, where I can actually leave a review of a restaurant or store I visit.
What are your thoughts on Facebook check-in feature and do you use it? Do drop in your comments.