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A hacker attack Thursday shut down the fast-growing messaging service Twitter for hours, while Facebook experienced intermittent access problems.
Twitter said in its status blog Thursday morning it was "defending against a denial-of-service attack," in which hackers command scores of computers to a single site at the same time, preventing legitimate traffic from getting through.
The fact that a relatively common attack could disable such a well-known Web site shows just how young and vulnerable Twitter still is, even as it quickly becomes a household name used by celebrities, large corporations, small businesses and even protesters in Iran.