Knowing where your friends are, and seeing their location are two very different things. Loopt's latest iPhone update, and soon-to-be Android update, illustrates this point nicely.
Today the iPhone version of the app is getting an update to finally include integration with Facebook's Places feature, which was rolled out to users in late August. Now Loopt users can check in to both networks at once, and see where friends are from both places on a single map.
Of course Loopt is not the first iOS app to map out Facebook friends (Sobees got there last month), but it's done it in a way that integrates with the company's existing social-mapping tools. You can, for instance, see where Facebook friends have been visiting in the past--something you have been able to do with Loopt buddies. This can end up being a nice way to discover new locations without a lot of legwork. The feature also ties into Loopt's friend alerts feature, so that you can get a ping when someone is within whatever range you specify.