Replying to Facebook Copyright infringement

I was just notified of a copyright infringement by facebook, because I used some songs in a video edit. How would the record labels... or artists have ever found out I used there music? It's not like the edit was massive and great... it was just some junk I threw together of some skiing. Kinda confused as to why they would even bother, but I probably should have atleast credited them for the tunes. But once again, it's intention was never to leave my small circle of friends since it was embarressing.