And think about how much this costs Twitter! The Fabric SDK allows apps to do phone number verification (SMS/Voice calls) for free. This is usually the single greatest cost associated with running any app that does phone number verification (often even above engineering salaries). Multiply the number of SDK users by the cost of SMS/voice delivery, and that's a ton of money out the door every month.

There were some rumors that Twitter has tried to spin off Fabric into another home, but it sounds like potential buyers balked when they saw the costs associated with it.

The prevailing theory is that Twitter originally saw this as a way to collect user data in the mobile app paradigm in the same way that the Twitter "follow" and "tweet" buttons do in the web paradigm. It sounds like it hasn't paid off, though.