I'm pretty sure you can do good even within facebook, doing your utmost to keep the company accountable (from my experience in another big corp, we don't see 1% of what's happening inside it, and how many people are facepalming - and we'll never know if many things were just humans being stupid or actual calculated decisions). You can also keep your guard up from outside and force facebook to fix itself (obviously, as much as its business allows) from outside, for example pushing it to hire more moderators and get as better so to prevent things like myanmar from happening again.
What I'm saying is that it's impossible (and in my opinion, pointless) to claim moral superiority and to accuse people of being morally bankrupt because they work for corp X.
There are plenty of companies which can't easily be categorized as having significant negative effects to the world. You can work for a "bad" company but consciously constrain your work to a business unit which improves customers' lives.
What you're using is false equivalence. You know what the worst thing for the environment is? Being born. Why do people insist on living when everything is bad? Reject the notion that you're powerless to change things.
Its like, "Doctor, why dont we just apply pressure on the tumor until it starts to grow at more reasonable rates!".
No. When you find cancer, you try to eliminate it.
Facebook is exactly this -- cancer. They have been aggressively monopolizing software for socializing so that they can arrive to the dominant position they are in now.
Until Facebook becomes more transparent w.r.t how they use the user data and until Facebook gives users autonomy -- they need to be regulated. We need to define constraints regarding how they present and manipulate user data and interactions.