. . .until it isn’t.
As this story suggests, Tweeting is a form of “publishing” and brings with it the responsibility not to make factual assertions about identifiable individuals that are libelous. Statements are libelous (at least in the United States) if they are
—false,
—damaging to the person’s reputation,
–and costly to the victim in some tangible way.
So, to everyone online, I say: welcome to the ranks of  “the media.” Check your facts.
This is why the news media, for all their faults, have fact-checkers, editors, lawyers, standards, and schools of journalism. You should know your song well before you start singing.
AN UPDATE: Here is a different view, from Jonathan Zittrain. It ran in the Financial Times, but I can’t find a free way to read it. (Good for them, not so good for me.)
Prof Chris Daly's Blog 