The data that Facebook collects about people goes far beyond what is explicitly shared and visible in their profile. E.g. which sites they visit (and when) with Facebook widgets on them, on-site browsing habits, private conversations, their phone contacts, location data, etc.
I imagine that a number of features are built on top of these. I remember that you could easily see what friends where nearby you when you were traveling (I ran into a friend who was visiting Milan at the same time as me a few years back!) but the feature doesn't exist anymore. I'm wondering if it's because of regulations that they had to cut down on these features.
