"In unveiling changes to its API rules last week, Twitter’s Ryan Sarver claimed that according to the company’s data “90% of active Twitters users use official Twitter apps on a monthly basis.” Twitter defines “official” as applications it owns and operates for the Mac, iPad, Android and Windows Phone.
It was an assertion that captured our attention because previous studies by Sysomos suggested there were significantly more “non-official” users. In a June 2009 report, for example, we found TweetDeck had nearly 20% of the client market.
To determine usage of different Twitter clients, we reviewed a sample of 25 million tweets on March 11 – the day Twitter unveiled its new API policy. The data shows that 42% of tweets were made by non-officials apps – more than four times the amount claimed by Twitter."
Source: Research by Sysomos, reported on their blog, 15th March 2011