But Twitter has a patent pledge with its engineers [1]:

    The IPA is a new way to do patent assignment that 
    keeps control in the hands of engineers and designers. 
    It is a commitment from Twitter to our employees that 
    patents can only be used for defensive purposes. We will
    not use the patents from employees’ inventions in 
    offensive litigation without their permission. What’s
    more, this control flows with the patents, so if we sold
    them to others, they could only use them as the inventor 
    intended.

[1]: https://blog.twitter.com/2012/introducing-the-innovator-s-pa...