As they put in their update, they aren't able to track 3rd party services. Considering 50% of all Twitter users use Twitter through 3rd party applications, that is probably a huge chunk.

However, I wouldn't be suprised if Twitter doesn't have a good retention rate. Especially if you don't go into it with friends, starting out on Twitter is a little hard, and takes time to build up your stream of information and relationships with other people.