Twitter goes by what it calls the Innovator's Patent Agreement (IPA), which is quite an amazing policy. The basic idea is that any patent granted for a technology that was built while someone is employed by Twitter will remain in the control of the engineer. Basically, any patent under the IPA can only be used defensively, even if sold to another company, and can only be used offensively under the express permission of the engineer or designer who filed the patent.
The IPA does apply to this patent, and so Twitter can only use it defensively. The patent came to Twitter because it was filed by Loren Brichter, founder of Tweetie, the day before Tweetie and Brichter were acquired by Twitter. Brichter has told Twitter that he only wants the patent used defensively, and so it shall be. And, even if Twitter wanted to break the IPA, the terms of the IPA state that Brichter reserves the licensing rights to the patent, so he could simply license the technology to whomever is being sued and nullify Twitter's suit.
Not a bad system, right?