[JDavo]

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...

[lmm]

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.

[sp332]

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.