3DS Friend Code Finder app on Facebook? It's a simple enough tool: Just input your code, which every one of your Facebook friends who accesses the app will be able to see -- and vice versa, naturally.
It's a pretty neat application, not only because it saves you the trouble of asking each of your real-life cohorts what their code is individually, but because it provides you with a surprising catalog of everyone you know that owns the system. For instance, Mr. Jeffries, your fourth grade biology teacher? He's got one, and man, does that dude love him some Nintendogs.