From this : https://developer.android.com/training/package-visibility
facebook packages are not returned anymore by GetInstalledPackages, so IsAppInstalled returns false and always uses webview.
To fix, you can put this in your AndroidManifest, within the manifest tag
<queries>
<package android:name="com.facebook.katana" />
<package android:name="com.facebook.lite" />
</queries>
It think it could be fixed in the package by using this
LoginManager.Instance.SetLoginBehavior(LoginBehavior.NativeWithFallback);
instead of
LoginManager.Instance.SetLoginBehavior(IsAppInstalled() ? LoginBehavior.NativeOnly : LoginBehavior.WebOnly);