"I wish I could say that today's outage could be explained by the Olympics or even a cascading bug," said vice president of engineering Mazen Rawashdeh in a blog post after service resumed. "Instead, it was due to this infrastructural double-whammy. We are investing aggressively in our systems to avoid this situation in the future."

On Thursday, users trying to log on to its website were greeted only by an incomplete error message: "Twitter is currently down for. We expect to be back in."

Twitter - infamous for its "fail whale" outage icon in its early years showing a whale held up by a flock of birds - last went dark for several hours on June 21.

The June episode revived fears that stability issues may once again be plaguing Twitter, which claims to have significantly improved its infrastructure. The company blamed that incident on a "cascading bug".

Founded in 2006, Twitter's phenomenal growth means it has struggled to handle the ever-rising volume of tweets. But in recent years, it has devoted considerable resources toward improving reliability in a move to project itself as a mature, polished brand.

Chief Executive Dick Costolo, who has focused on improving the service's profitability and attracting advertisers, said last month that Twitter has 140 million active monthly users who send 400 million tweets daily.

Google Inc's online messaging and voice-calls service - Google Talk - also went down for hours on Thursday in what appeared to be a separate outage. The issue should have been resolved as of mid-morning, according to the company's status update page.

(Reporting by Gerry Shih; editing by Lisa Von Ahn, Richard Chang, Andre Grenon and Andrew Hay)

• A Permanent Economic Emergency