I'm not so sure. What direction will it "sure move" if you short 20% of FB float? You have to borrow a stock to short it from someone, so the stock that you short is not actually sold just lent.
The price would definitely move down if you sold 20% of facebook's float. The market doesn't care that you borrowed and are the one selling (as opposed to the party you borrowed it from selling them), all that matters is that 20% of the shares are being sold.
Hedge funds, which can turn huge sums into market swings often use this technique to drop the price, creating additional drops from panic sell offs, allowing them to make a margin on it from the bottom.
Is that legal?
It's lent but you just artificially increased the volume because otherwise someone would have to sell actual shares.