Twitter announced Wednesday that it had bought TweetDeck and pledged to continue to invest in the popular application for using the micro-blogging service. "We're pleased to announce that the TweetDeck team has joined Twitter," Twitter chief executive Dick Costolo said in a blog post. San Francisco-based Twitter did not disclose the financial details of the acquisition, but various reports in technology blogs have put the purchase price for the London-based TweetDeck at around $40 million. "TweetDeck is a great example of a third-party developer that designed tools for the incredibly important audience of Twitter power-users and, in turn, created value for the network as a whole," Costolo said. "This acquisition is an important step forward for us," he said. "TweetDeck provides brands, publishers, marketers and others with a powerful platform to track all the real-time conversations they care about. "In order to support this important constituency, we will continue to invest in the TweetDeck that users know and love," Costolo said. TweetDeck, which was founded in 2008 by Iain Dodsworth, is a favorite of heavy users of Twitter, allowing them to view "tweets" in various different ways and to organize their messages into columns, features which are not offered on Twitter's own website.