It is cool to say that Facebook is doing 'world-changing' and 'revolutionary' things -- while I believe such is true on some levels and that we can't ignore what it is -- but a company like Plantir has a much better opportunity to do things that are simply mind-boggling.
Finance and the Government are not popular things, especially amongst the hacker crowds, but the US Government is capable of doing incredible things. The power wielded by the State is huge, and directly affects hundreds of millions of people a day. Facebook may have become a staple in some peoples lives but they [as a company] are not responsible for keeping citizens safe, regulating commerce, building infrastructure, waging war or brokering peace.
Facebook is a company that finds better ways to serve up advertising. That's not all they do, but that's how they make money.
Plantir is in a unique position to change an increasingly inefficient/ineffective system. US politics suck right now. We can all agree on that. But a company like Plantir operates on a different level than the fork-tongued politicians.
You say: network with people in the startup industry and create my own startup which tells me (and I might be wrong, please correct if so) that you enjoy the idea of disrupting the status quo. What better place to do this than Big Government and Big Business/Finance? This might seem backwards but as these institutions crumble at their seams there is no better place to be.