This is a perfect example of how web applications should work: by gathering data from different places and making logical conclusions.
Last night I attended a friend's surprise party where I met a new friend. We became friends on Facebook during the party. Because we both RSVP'd to the Facebook invitation, Facebook was able to gather that we met at the party and became friends shortly afterwards as a result.
This is how all web applications should be: much more human-like and logical. Kudos to Facebook for building in this kind of logic.