I have no problem visualizing how Target is able to predict future purchases based on what items I've already bought or what items I'm browsing. I have trouble visualizing how anyone can predict future purchases based on me liking a picture of a cat or browsing through friends vacation photos. Unless they're planning on selling me cat vacations...
Even your age, sex and country is very useful to advertisers - an ad targeted at those is worth about 50x the price of one sent to a random internet user (or it was when I last worked in that industry). Facebook knows where you went to college, what your favourite movies are, and perhaps most importantly of all, who your friends are. That's huge.
They just have to correlate: people who browse cat photos at 3 am and "like" these specific ones tend to buy canned beans within the next 3 days. It doesn't have to make sense as long as they can derive correlations from it.
