I acknowledged that possibility myself, but frankly, I think most people just won't bother. They don't seem fazed about giving Facebook a ridiculous amount of personal information about themselves, and other similar sites.
Those who feel a burning need to do so, of course, will, but there's little we can do about that, regardless of whether it's on the Internet or not. The kind of people making these threats aren't moustache-twirling genius supervillains, though. I'm talking about social commentary sites (yes, like forums) having significantly less info on you than Facebook already has; this isn't some huge sacrifice of privacy or something.
Will there be ways around it? Sure. There's ways to get away with murder, too. I'm just suggesting we have even a modicum of systems in place to take action on criminal threats. Threatening someone over the Internet shouldn't end by being banned from the site, it should end with a visit from the authorities and a potential arrest. The same way it would if you made the same threat over the phone.