Next evolution: use their recognition algorithms to identify shirts, jewelry,... in your friends photos and have "buy now" buttons all over the picture.
I actually think that could be a decent way to balance the fact that people don't want to see ads with the fact that FB has to have ads in order to make money. As long as they did it in a way that didn't disrupt the user experience too much
Unsurprisingly enough, that's already patented, as far as the image mapping to purchase process is concerned. I believe by Amazon, though.