This proves two things:
1) Nothing on Facebook is private. If you even think it is, you're an idiot. They will find out, track it, etc. That's why I got off Facebook when they changed their privacy policy a few years back. Don't need it. And yes, the FBI, etc. has sniffers to find this type of stuff specifically. This, political threats, national threats, etc. And not just on Facebook...
2) Some idiots really deserve to get busted. Seriously?!? Let's just threaten a bunch of kids. In what universe is that ever cool?!? Ever?!? Anywhere?!? You double-dumb-@$#... Sit and rot... Turning yourself in should just possibly prevent the needle, but you should still sit for more then 2 years IMHO.