Facebook has shown a willingness to send Cease and Desist letters to people who do this[1]. I'm sure they would be more than willing to follow up with a lawsuit. Most sites (that I've actually looked at :) with APIs have clauses against the automated gathering of data from their site. This would fall directly within that.

I suppose if you are located somewhere where you feel like you are outside of the reach of their legal arm, you might be able to get away with it. Otherwise, you are in a legally shaky position, at best.

1. http://www.zdnet.com/blog/facebook/facebook-bans-browser-plu...