I would not call myself a "facebook" wiz, but I know several people who are. I no way whatsoever am I telling to not take a threat seriously, but I have no doubt that the software is out there to send mass emails / friend requests / messages etc. to random people on facebook. Again, if you have reason to believe that the threat is someone that you actually know in person, what I'm saying may not be applicable, but it may very well be the case that over 10,000 other facebook users received the same threat.
As far as the "tracing who sent it", I don't know of any way right off hand to figure this out, but again wouldn't be surprised if there were a way. If you received this email through your regular non-facebook email, there are ways to ultimately trace this information back, but I'd say there's a fair chance that it was sent from an anonymous account with fictitious user information. Nailing this down to a person in such a way that it will lead to someone being put in handcufffs is often times cannot be done by one's self, although it sure would be nice if there were an easy way.
With regard to calling the police, I certainly don't blame you. However, it has been my observation that the police tend not to take this all that seriously because it is something that is probably a bit difficult for them to investigate and prove (as opposed to having observed a robbery caught on security camera, etc.) This is much the same as the fact that the phone company is more able to do something about random phone threats than the police themselves.
What you might do is contact facebook themselves. You don't actually have to have a facebook account to do this, and they may be more able to do something about this in a direct way than the police, or whomever else. The website for reporting such things is:
Hopefully, this may help, although I imagine that facebook receives many more "threat reports" than they can respond to on an individual basis.
let me know if there is any outcome to this.