We encourage you to communicate with your members wherever they are. This means that if most of your members are active on Facebook and not using other social platforms, it may make sense to use a Facebook group to communicate with your members. Other possible options are Google Groups, Google+, etc.
Ultimately, the channels you use (and don't use) to communicate with your groups is completely up to you. Go to where your users are.
But you require GDGs to set up a Meetup.com page?
It is true that we require all GDGs to create a Meetup.com chapter page during the onboarding process. However, this is to ensure that we use a common platform to track our GDGs and gather their basic information. It is these Meetup.com chapter pages that allow us to centralize chapter information into our chapter directory. Without a common platform, and a common account system to log into developers.google.com, managing (or even locating!) GDG chapter basic information would be impossible.