As an engineer you need to stay where there is green. Big companies like facebook don't need you. They are the big oak trees and you're a little green leaf trying to sustain all that mass of branches and trunk beneath you desperate to soak up whatever energy you photosynthesize. Facebook may want you for lots of wrong reasons, but they don't need you. You want to work somewhere where you are absolutely necessary for…
Eh. Some days I'm just out of it. I can promise you I can do the heroics tomorrow and solve the problems nobody else can solve, but today I just didn't get enough sleep (or whatever) and I'd like to go through my backlog of low-mental-energy tickets for a bit, thanks. If in a few years you decide you don't need me, that's fine; I never defined my identity in terms of you anyway, and I can go somewhere else.
I've done the thing of being absolutely critical for a startup. It leads to things like being on 6 AM debugging calls with a customer around the world, getting urgent calls when you're on vacation, etc. If you enjoy it, by all means, go for it. But it's not universal to enjoy it, and there are plenty of jobs where you can provide a ton of value between 9 and 5.
There are plants that thrive in the middle of open fields; there are plants that survive on the rainforest floor. What's good for one isn't necessarily good for the other.