Before I begin, I'm not trying to troll you and am certainly not trying to be disrespectful. However, cashing out $1B of stock and buying a company (which could turn into a serious competitor) for $1B have absolutely nothing to do with each other.
The sale of stock was about liquidity, diversity and paying off taxes. It was the right financial decision (that any advisor worth his/her fees would recommend).
Finally, Mark Zuckerberg did not buy Instagram. He negotiated the sale on behalf of a company which he happens to own a controlling interest in. Legally, Facebook and Mark Zuckerberg are different entities.
