Mark and Systom already know each other even before Instagram was created. The duo had been seeing each other for a long time. Mark even asked Systrom to join him at Facebook. So now you know the great bond between them. I believe that's what made Systrom to sell Instagram to Facebook, instead of selling it to Twitter which had already made a formal agreement.
Back on the deal, Systrom came back to Mark to sign the deal officially. But what he saw baffled him. Mark was busy himself and Friends watching HBO ''Games of Thrones''. This gentleman could throw a billion dollar deal under the ocean without feeling a thing. As for Systrom, he had to wait outside and did phoned lawyers and investors to sign off the deal.
