We were at NYC last week voting One Show Interactive (I still havel to update that post!) when Iain Tait came up with one of the most funny and memorable quotes of the event: "i like this piece very much, it's not about being so-well crafted or being a great production at all, i just like it simply because it is so fucking awesome"... which prompted us into a big laugh.
Then we saw the work, and I think that this is one of the most remarkable uses of Facebook I've found out this year. Done by Daddy to promote Telia, an online music store in Sweden, it uses the platform in such a naive way.
Basically the whole thing consists in sharing one song from the Telia catalog with a friend of yours in Facebook, and then the app generates a slideshow of the pictures in which you and your friend are tagged, zooming that square area that defines the tag itself. This may sound silly, but it's hilarious when you experience that, for instance, me sharing romantic theme "Nothing compares 2U" with my friend Pit, because that is indeed the way in which stuff at Facebook is shared, so in the end the app is so native to the platform that although i have no stats of use i'm pretty confident that it did pretty well.
You can use the app here or read and see further examples in this link from Daddy.