Putting up an interesting post about Twitter mobile usage, the microblogging site’s chief executive Evan Williams disclosed on Tuesday that his service currently has 145 million registered accounts and nearly 300,000 apps using its open API.

Williams also listed the top 10 ways in which Twitter is currently being used by people - both in terms of the Twitter’s own apps as well as third-party clients like TwitPic, TweetDeck, UberTwitter, Echofon, and Google Friend Connect.

However, the post downplays the rather surprising fact that Twitter for Android does not feature among the listed top 10 ways, even though Twitter for iPhone and Twitter for Blackberry feature at the fourth and fifth spot respectively.

About Twitter’s decision to create or buy its own clients for different computer and mobile platforms, Williams justified: “We did iPhone user tests and confirmed that even though there was a plethora of third-party Twitter apps, people were having trouble finding and selecting one because none were called “Twitter.” This kept them from using Twitter at all. For this reason, we acquired Tweetie and turned it into Twitter for iPhone, complete with a new user sign-up experience.”

Despite the fact that Twitter goes all out to ensure that its developers still feel loved, more so as the site’s widely-used API laid the foundation of its success, investor Fred Wilson remarked a few months back that it is one ting to build apps that expand on Twitter and quite another to fill product gaps that Twitter could not get around to earlier on!