I recall a similar feature being in the Facebook app about four years ago, but it relied on your friends to manually check in. This new interpretation is more like Find My Friends insomuch as it’s a passive location feature. Unlike Find My Friends, though, it appears to only require confirmation on one end of the exchange. That is, if you’ve enabled Nearby Friends, all of your Facebook friends can now see your location unless you’ve explicitly blacklisted them. It’s only available in the US right now, but expect it to roll out quickly if Facebook decides to run ads against your location.