NEW YORK (AP) - It's commonly used by celebrities, big companies, protesters in Iran and ordinary Americans to get their messages out to friends and the public.
But the vulnerability of Twitter to a hacker attack has been revealed, after an attack that shut down the messaging service for hours today.
There were also some access problems at Facebook.
In its status blog, Twitter said it was "defending against a denial-of-service attack." That's where hackers command scores of computers to a single site at the same time, preventing legitimate traffic from getting through.
One New York City public-relations manager says she relies on Twitter for news and felt "completely lost" without it.