Twitter went down between about 8:20 a.m. and 9 a.m. Pacific Time on Thursday. Two data centers that operate in parallel for redundancy both failed, the company says. The outage lasted as long as two hours for some users. It was Twitter’s second outage in about six weeks, with a cascading bug that spread from one software element to others. The company says it is investing aggressively in its systems to avoid this situation in the future, an official says.”]