Facebook is going to give advertisers that want to target its users even more ammunition. The social network will start letting advertisers take information about what Facebook’s users do on the site, mix it with data about the stuff they do on other websites and use it to hone their pitch. Facebook calls this “interest-based advertising,” and says that this is common throughout the Web, which is true. It’s also what Facebook said two years ago it wouldn’t do. Back then, Facebook began letting advertisers “retarget” its users based on their travels outside of Facebook. That is: If you visit Zappos before you come to Facebook, there’s a good chance you’ll see a Zappos ad while you’re looking at photos of your friend’s dog. But Facebook was emphatic that advertisers couldn’t combine that information — stuff lots of people could find out — with information about what you did when you were inside Facebook — stuff that only Facebook knew.