Facebook chose to make Facebook Login spyware, and Spotify didn't think anyone would mind when they did, because who cares about tracking? and/or wasn't aware, because who could realize this from the unclear Facebook docs? (Either is possible!)
That was a very pre-2019 decision, and now that Facebook just set the world on fire, we're probably going to see a lot fewer Login with Facebook.
The ironic thing is that you can actually implement Facebook login in a privacy-respectful manner by using standard oAuth where Facebook is only contacted when the user decides to log in with it (which is mostly fine as they have an account anyway and thus consent to Facebook tracking).
It's likely that apps that load the SDK are using other features of it, such as Facebook Analytics (which is similar to Google Analytics), not just login with Facebook.
That was a very pre-2019 decision, and now that Facebook just set the world on fire, we're probably going to see a lot fewer Login with Facebook.