When the White House abruptly announced an address to the nation by President Obama on Sunday night, CNN anchors spent the better part of an hour previewing the address without actually saying what it was about.

“I have my own gut instincts on what it might be,” the CNN anchor Wolf Blitzer said a couple of times, adding that a senior White House official had thanked him for showing “restraint” and not speculating.

Thanks to Twitter and Facebook, some CNN watchers had already heard the news. Unconfirmed reports — that turned out to be true — of Osama bin Laden’s demise circulated widely on social media for about 20 minutes before the anchors of the major broadcast and cable networks reported news of the raid at 10:45 p.m., about an hour before Mr. Obama’s address from the White House.

It was another example of how social media and traditional media deal with the same news in different ways and at different speeds. Just as CNN once challenged newspapers and evening newscasts with a constant stream of images from the Persian Gulf war, Twitter and Facebook have become early warning systems for breaking news — albeit not always reliable ones.