A post from VentureBeat by Owen Thomas explains:
Twitter CEO Ev Williams posted a blog entry Thursday with an update on the microblogging service’s usage, which continues to grow sharply. More interesting than the numbers, though, was the message written between the lines: Twitter now has 145 million monthly users, and for the marketers who want to reach them, Twitter is the only way to go.
The numbers don’t lie. Third-party developers, who played a large role in Twitter’s early growth and buzz, just don’t matter.
The passive-aggressive blog post is a Silicon Valley art form, and Williams, who helped push blogging into the mainstream with his earlier startup, Blogger, is surely a master of the medium. A chart Williams includes (pictured below, with a few annotations from yours truly) tells the story: After Twitter introduced its own iPhone and BlackBerry apps, on top of its own website, SMS, and mobile Web versions, it now controls the top five most popular ways people access Twitter. No third-party app amounts to more than 4 percent of Twitter’s audience — and the most popular of those is TwitPic, a photo-sharing service.