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.
Google is actually just the 6th largest content network. If you want to truly extend your http://www.adroll.com campaign to the fullest, it helps to display your ads across multiple content networks - as opposed to only one.