Kristina P, writing on the official Jetpack blog, in a post titled “The End of Twitter Auto-Sharing”:
Twitter decided, on short notice, to dramatically change the terms and pricing of the Twitter API. We have attempted to work with Twitter in good faith to negotiate new terms, but we have not been able to reach an agreement. As a result, the Twitter connection on Jetpack Social will cease to work, and your blog posts will no longer be auto-shared to Twitter.
This was bound to happen, and we’ll be seeing many more such announcements in the coming days.