fb.js is Facebook’s standard JS base, with things like polyfills/ponyfills to ensure certain features in a browser environment. It’s imported by React, Relay, etc.

So this might be what you’re seeing, but normally it’s included in a precompiled JS application bundle.

I believe you're thinking of FBJS (https://github.com/facebook/fbjs), which is a library as you describe, whereas the comment above is referring to loading the Facebook SDK from Facebook's servers. Among other things, Netflix offers Facebook login, which would need the SDK loaded.

Don't they use React for their UI?

Surely they would serve it from their own CDN though?

Taking advantage of everyone having it already cached on their machine maybe? Or it could just be standard ad retargeting - not unreasonable that Netflix would want to stream behavior data to facebook ads for targeting / lookalike purposes