settings.gradle must define all the modules. It won’t load other settings.gradle found in the tree to load more module.
You’ll have to either
define a module for the facebook SDK in the top level settings.gradle. Yes it’s redundant with the other settings.gradle.
publish Project B somehow (as well as its dependencies so in this case the facebook SDK library), somewhere (a corporate artifact repository for instance) and access it from Project A.
While #1 is better, it makes the ProjectB -> Facebook dependency tricky as the path will be different depending on the settings.gradle used. One way to fix this is to split the module name/path from its actual location on disk. this is done inside the settings.gradle file.
In your top level settings.gradle file, do
include 'facebook-sdk' project(':facebook-sdk').projectDir = new File('Libraries/ProjectBRoot/Libraries/FacebookSDK/facebook')
In the setting.gradle file inside your Project B, do the same with the different relative path:
include 'facebook-sdk' project(':facebook-sdk').projectDir = new File('Libraries/FacebookSDK/facebook')
This makes both project setup define the same ‘facebook-sdk’ module located at the same absolute location on disk.
All projects depending on this module should just declare the dependency as