You can have four modules inside your project
Module 1(Facebook)
Module 2(Twitter)
Module 3
Depends on Module 1 and Modules 2
Main Module
Depends on Module 3
in future if you want to include another module in your project you will not have to change build scripts of your main module.
EDIT :
Now your Module 3 is an independent module which you can use anywhere you want to integrate all these sub-modules like Module 1, Module 2,...so on.
I have never tried but you can refer this link if you want to use same module among multiple projects or if your Module 3 doesn't lies inside your project directories :
Android studio add external project to build.gradle