If that were as straightforward as you claim, there would be a black market for all sorts of serverside vulnerabilities that might swing stock prices. But there isn't. One of two things is probably happening:

1. It is way less easy to predictably and profitably swing Facebook's stock than simply by hacking 10 celebrity pages.

2. For whatever reason, including the fact that crime rings premised on manipulating stocks have an annoying tendency to get caught, nobody is running this "hack Company X while sorting their stock" scam, and so even if it's possible to accomplish, the market doesn't value it.