Yammer co-founder David Sacks announced he’s leaving Microsoft on Wednesday, tweeting a thank you and goodbye to his former charges, whom he referred to as his “YamFamily.”

Microsoft acquired the maker of the enterprise social networking and chat tool two years ago for $1.2 billion, 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.

Yammer will be moving under Office 365 and Outlook as Microsoft looks to further integrate Yammer technologies throughout its Office productivity suite. Kristian Andaker, an Office team lead, will be heading up Yammer engineering, ZDNet’s Mary Jo Foley reports. It’s not clear what that means for Yammer cofounder and CTO Adam Pisoni, who thanked Sacks for the “wild ride” the two had together:

Prior to joining Microsoft, Sacks cofounded Geni, a genealogy website, from which he spun Yammer out, and was chief operating officer at PayPal. 

Lead image by TechCrunch on Flickr