I think the app would really please the people who put stickers on their photos (like with the messaging app LINE). It's also great that it isn't a required download, so people don't *have* to use it.
I'm not a huge user of FB, but I quickly learned that if I use the browser version on mobile (in Safari) the content-blocking rules apply whereas they don't if in a FB app.
Is there something you can't do on the mobile website?
I also have a dummy facebook account with a standolone dummy gmail address that is solely for games or apps that require a facebook account for signup, savegame, free doohickies, etc. Using only mobile makes this easier.