Yammer co-founder David Sacks announced he’s leaving Microsoft on Wednesday, tweeting a thank you and goodbye to his “YamFamily.”Microsoft acquired the enterprise social networking and chat tool two years ago, and when it did, Sacks was seen as a potentially transformative figure for the legacy technology company. He was even mentioned by outsiders as a candidate to replace Microsoft's outgoing CEO Steve Ballmer last year, though it’s not clear if he was ever seriously considered.