Steemit is just a tool to access Steem the blockchain. STEEM is the currency native to Steem and the market cap of STEEM is currently roughly 1,6 billion dollars. The market cap of Facebook is currently about 40 billion dollars. Zuckie would have to buy out enough of the whales to take over the code. He could do it but if he did it just to bury it, he'd have to keep buying up every similar blockchain that came after it. It's decentralize or die.