[gradys]

Well, Facebook can tell how often you interact with certain people. They can determine certain things about the content of those interactions. They can detect how similar your friend graph is to another person's. They know that your grandma is your grandma (if you have told them, of course), and they know, for example, that most people don't want to hang out with their grandma.

I haven't given you pseudocode for an actual algorithm, but I can imagine that Facebook could combine all of these metrics into an index that can tell them roughly how likely you are to want to hang out with certain people.

[angryasian]

without you telling explicitly who you want food recommendations from or tech product recommendations from its an incredibly difficult problem. Just because you talk to someone everyday doesn't mean you want their opinion on food or tv shows.