[michaelt]

  Somehow I feel like the people that are serious about this
  level of criminal activity, and capable of actually 
  accomplishing it, aren't first announcing their intentions 
  on twitter.

If you're in the secret service and you let the president get assassinated, it would be much more embarrassing if the assassin had announced their intentions in public.

Once the assassin has been identified, if they had a tweet from a month ago saying "I am going to assassinate the president in a month" you can bet that will be on the front page of every newspaper, making you look like an idiot.

And if your anti-embarrassment system should prevent a real assassination, so much the better.

[mmmooo]

Though I completely agree with the intention of your point, the same sort of logic can be used to justify all sorts of dubious "three letter" activity.

By that same logic, are you still an idiot if instead of a tweet, it was a facebook public post? How about a facebook post marked only visible to your friends? Or in an email to your brother's wife's third cousin?

I'm not a big fan of slippery slopes, but this one looks a bit lubricated.

[michaelt]

The criteria for it getting on the front page is "anything an investigative journalist could get their hands on with a few days of dedicated searching, assuming they knew the person's name"

So tweets are in and public facebook messages are in, but private facebook messages and e-mails are out.