A technical glitch blocked or slowed access to Facebook for several hours Thursday, unplugging many of the social-networking site's 500 million users from the constant flow of updates from their friends. Facebook called it “the worst outage we've had in over four years” in a blog post published late Thursday.

The company blamed a software flaw that caused a cluster of its databases to be overwhelmed. Facebook said it had to shut down the entire site to stop traffic to the databases. The company estimated the problems persisted for about two and a half hours.

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