It's not publicly discussed for obvious reasons, but I worked closely with Mudd for a couple of months last year. Here's a related interview where he touches upon it.

http://digiday.com/platforms/advertisingweek2015-facebook-fi...

Facebook spend an awful lot of resources tracking all sorts of user behaviour inside it's "walled garden".

I got really sick of it after a while and decided to stop working for them (even as a contractor)