(1) Twitter adds 250 million new users that use it regularly.
(2) Facebook copies all of Twitter's features.
As I wrote in March, it is not difficult Facebook to copy all of Twitter's features, and they have since shamelessly done so.
Think of your stay-at-home mom in the midwest that is using Facebook to keep in touch with all her friends. She wants to subscribe to businesses to get deals and see what her favorite TV stars have to say. What's she going to do? Try out Twitter with its lack of inline photos and videos and unintuitive nonthreaded comment streams, or become a fan of their Facebook fan pages and have everything cleanly inserted on her Facebook home page? No brainer.
As reported in Techcrunch, Comscore shows Facebook continuing to grow while Twitter is flattening out:
Twitter is great and all, but it needs to fix its UI ASAP if it wants to grow. And no, expecting moms in the midwest to find random 3rd party applications does not count!