If Apple aren't going to make it possible to use Facetime on Android or Windows devices, they should shut it down. Using Facetime or iMessage groups is anti-social, because it excludes 70% of Europeans, 40% of Americans, and 85% of people globally who don't have iPhones.
All these apps should be using open protocols even if they have proprietary clients. Proprietary protocols for common apps are almost always motivated by a desire to create barriers to market entry.