It’s called ad re-targeting, and it’s the most effective innovation online ads have seen in a while. And no one will be able to do it better than Facebook. Well, Google already does it better, but Facebook might be able to catch up.
By contrast on Facebook it's just a vanity contest.
One example ... I'm sometimes searching for interesting articles in an area, but without having anything specific in mind. Delicious provides way better results than browsing Reddit ... that's because on Delicious the rating is not a conscious choice.