That’s most likely the Facebook SDK we’re talking about. The SDK phones home regularly regardless of whether it’s features are actually being used.

You can get NetGuard on Android and see how often and what is called (facebook.com, graph.facebook.com, other vendors, platforms, IPs, etc). You can choose to block what is not needed. Kind of uBlockOrigin for Android. Sometimes stuff breaks.

And a lot of stuff is not related to FB Login.

Facebook launched Facebook Off Activity a while ago https://news.ycombinator.com/item?id=22178917

You can go to your profile, and check which third parties, or advertisers uploaded contact data of you, download backup data to see in json files which apps what sent about you ("App activated", "Made some purchase").