During his Congressional testimony, Zuckerberg was perched on a booster cushion to, well, give him a boost.
In the long hours of testimony he became very attached to the booster cushion, so much so that when his testimony was over, he refused to surrender it to Congressional staffers.
When they tried to grab it from him, he clutched it more firmly. Then he launched into a kicking-and-screaming tantrum on the floor. "Get away from me, you're not my friends," he shouted at the staffers trying to grab the cushion.
One of his handlers was able to calm him down. "Now, now, Mr. Z," he said. "Be a good boy and hand over the booster cushion to these nice folks. If you do, I'll give you two prizes from my attaché case."
So that's how Zuckerberg came to leave the hearing without the booster cushion but with a big lollipop and his favorite blue blankie from home.