It also knows about 1 trillion connections between people, pages and so on
Facebook's Graph Search is very much in the early days, it's got a few hundred users at the moment. But there is potential and the big reason for that potential is the sheer amount of data Facebook is sitting on. For example, there are now 240 billion photos uploaded to the site.All the top photo sharing services combined don't even come close, they've got a few billion at most each, Facebook doesn't really have any competition in this department.
Users are adding billions more each month and this while Facebook Photos is rather limited. Once Facebook figures out a better way of including photos into the mobile apps and maybe merge Instagram in some way, the number should grow even faster.
Photos don't actually store that much meaningful data, for a computer, but the average user has 200 photos and over 1,000 connections.
In total, Facebook knows about over 1 trillion connections, i.e. friendships, page likes and so on. That's a lot of data with a lot of relevance, the problem is how to get the most or anything out of it. That's what the Graph Search is for.