One of the internet’s most popular networking sites is up and running again after crashing in what’s believed to be a pre-planned attack.Twitter users were greeted by a message saying “Denial of Service.” The site said the crash was caused maliciously in a coordinated attack, as hackers overwhelmed its servers with communications requests. The volume of traffic was so high the site could not cope. Twitter was not the only site affected. Facebook was also targetted, although it did not crash. The cyber foul play is the latest in the last few weeks to disrupt a number of South Korean and American sites, including that of the White House in Washington. South Korea has said North Korea is to blame. Twitter has boomed in the past 12 months. Its traffic is now 15-times higher than it was a year ago.