It's called Path and it's a new social network with a couple of very big differences to Facebook. First, it's more of a photo sharing service than a text/link operation. Secondly, you're limited to 50 friends. The people you pick don't necessarily have you on their list of 50 friends. The friends must be pulled out of your iPhone contact list (Android and Blackberry versions are on the way but this is a San Francisco startup so iPhone dominates) and those are the ones that can see the items you put into your feed or path or whatever terminology we're supposed to learn with this one.
You can tag each photo--with people, with a place, and with a "thing" in it. The entire item is what Path calls a "moment." There are some unique quirks to the service: the list of contacts you see in Path isn't the people with whom you share your "path," but the people who have opted to share with you, a marked (if potentially confusing) contrast to the voyeuristic world of Twitter. When you tag a "thing" in a Path "moment," you can type in anything but are also offered a list of other things that have been tagged nearby as well as at similar hours of the day.