Cyber attacks hammered Twitter, Facebook, and Google, disrupting the hip micro-blogging service and causing stumbles at the hot social-networking site while Google fended off assaults.
Twitter was down for more than an hour early Thursday morning, before the eponymous California firm got it back online.
"The continuing denial of service attack is being mitigated although there is still degraded service for some folks while we recover completely," Twitter co-founder Biz Stone said in an official company blog.
"Twitter has been working closely with other companies and services affected by what appears to be a single, massively coordinated attack. As to the motivation behind this event, we prefer not to speculate."
By late Thursday afternoon, Twitter said that service was improving but still sporadic with some people "unable to post or follow from the website."
Facebook was "degraded" by an early-morning distributed-denial-of-service (DDoS) attack on the Palo Alto, California-based Internet star's website, said Facebook spokeswoman Brandee Barker.
"No user data was at risk and we have restored full access to the site for most users," Barker said. "We?re continuing to monitor the situation."
Twitter and Facebook have teamed with US Internet powerhouse Google to investigate the attacks.