Slightly creepily, Tsarnaev's twitter account is still available for viewing. And you know what? His tweets sound... normal. Indistinguishable from the tweets of thousands, millions of other people. He tweets about pop culture, about what he's eating. He uses slang like "dawg" and "da bomb". There is nothing on there that suggests he's planning to blow up a whole bunch of people.
Now take Paul Chambers, who posted "Crap! Robin Hood airport is closed. You've got a week and a bit to get your shit together otherwise I'm blowing the airport sky high!!" This led to him being arrested, charged, and found guilty of making threats. This ruling was only finally overturned on the third appeal. The conviction lost him his job. He had no intent of actually blowing up that airport. He was making a joke because he was worried that he was not going to be able to fly out to visit his girlfriend.
A lot of people, especially non-Twitter-users, thought that he'd been very foolish to make that kind of joke publicly.
But was it done publicly? If he had made the same joke in private, let's say at the dinner table, no one would have thought it was a serious threat. On the other hand, if he had written a letter to the local newspaper, or stood on a soapbox in the town square and shouted it, it would come as no surprise if the police took at least some interest.
But is Twitter public? The assumption the law made, and the assumption a lot of non-users make, is that it is public, because everyone can read the tweets.
Now let's say Chambers had been walking down the road with some friends, and had made this comment to them. Not in a voice loud enough to be heard far outside the group it was intended for. Still, a passerby might have heard it. Would this have qualified Chambers for a visit from the police? Of course not. He would have been saying this to his friends. He would have been in public, but his joke wasn't for the public.
This isn't a distinction really recognized by the law, but it's one that is - or at least used to be - quite well-understood. There's a difference between saying something and announcing it to the world.
And I think this is how most of us use Twitter - as this in-public thing not intended for public consumption. We just use it to talk and joke to each other. But the powers that be don't understand this. They want to treat every tweet as if it were a letter to the editor, as if it were shouted from a soapbox on the street corner. They want to do this because they think that if they just snoop on our conversations thoroughly enough, they'll catch the bad guys.
But, listen: They didn't catch Tsarnaev in time, because if you plan to blow something up, you don't idly tweet about it. All they caught was poor Paul Chambers, and they put him through the wringer.