What's probably slowed adoption of that approach is too much "web thinking" in the app design process and too much "But what does Google / Facebook / Twitter do

What's probably slowed adoption of that approach is too much "web thinking" in the app design process and too much "But what does Google / Facebook / Twitter do?" In fact Twitter provides SMS based signup as a free service for app developers as part of Fabric - https://get.digits.com/ https://get.digits.com/