Microsoft Corp admitted its largest acquisition in the Internet sector was effectively worthless and wiped out any profit for the last quarter, as it announced a $6.2 billion charge to write down the value of an online advertising agency it bought five years ago. The announcement came as a surprise, but did not shock investors, who had largely forgotten Microsoft’s purchase of aQuantive in 2007, which was initially expected to boost Microsoft’s online advertising revenue and counter rival Google Inc’s purchase of digital ad firm DoubleClick. Microsoft’s shares dipped slightly to $30.28 in after-hours trading, after closing at $30.56 in regular Nasdaq trading. The world’s largest software company said in a statement that “the acquisition did not accelerate growth to the degree anticipated, contributing to the write-down.”