Fabric will be free, and designed to make it easier for developers to integrate Twitter tools when they build their products.
It will help developers do things like address bugs, by integrating tools from Crashlytics (which Twitter bought early last year), and insert mobile ads using MoPub (which Twitter bought a year ago).
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Another new service, called "Digits," enables users to log in to an app by linking that login ID to their phone number. The idea is if it's easier for users to log in, they'll be more likely to try it out, and they'll be more likely to return to it if they can identify themselves with their phone, rather than having to remember a user name and password.
Though Twitter isn't charging for Fabric, there are obvious revenue implications. The company could eventually charge for a premium version later, for example. And most importantly, if developers integrate Twitter's tools, like MoPub to insert ads, and feature more tweet streams in their apps, that should translate to higher revenue.