Twitter, if anything, prevents information overload to its users, giving them a limit to the information posted, but at the same time (and ideally), succinctly capturing the thoughts of the user for that particular time. "What are you doing?" is just a catalyst, a platform from which users can take on to 'launch' and begin their throught process, and come up with an interesting, informative, and/or thought-provoking tweet. Twitter and tweeting in today's media culture provides an alternative to users who are either tired or cannot pay attention to numerous and lengthy information posted online. At the same time, it provides convenience to users as they can still share their thoughts to other people without requiring them to come up with a lengthy...

Moreover, the lack of other graphics apart from the Update section and tweets from friends/people the user is following make online publishing in Twitter lighter in terms of byte transfer and more "ambient" -- that is, can be accessed through the Internet, mobile phones, and instant messengers. Indeed, as the Twitter creators have claimed, "getting in connected" in Twitter is possible because it is fast, succinct, and 'byte-light,' important features that reflects today's new media culture, a large portion of people who prefer on-the-go, available, and critical information anytime, anywhere.
Reference:

Twitter, official web site. www.twitter.com.