Sure he could walk away with $19 billion and was his hands of it.

once he agreed to take that money, he could no longer keep his promises, even if he wanted to.

Did he really believe FB wouldn’t try to maximize returns on its investment?

He could donate some money to worthy causes if it would help him feel better.

anyways it’s a “free” service so he doesn’t owe anybody anything. They could switch to another app. Like Viber if it’s not as intrusive.