During the first couple of days of the test, everyone wanted to tweet the full 280 limit because it was new and novel, but afterwards most people tweeted below 140 most of the time.
We – and many of you – were concerned that timelines may fill up with 280 character Tweets, and people with the new limit would always use up the whole space. But that didn’t happen. Only 5% of Tweets sent were longer than 140 characters and only 2% were over 190 characters. As a result, your timeline reading experience should not substantially change, you’ll still see about the same amount of Tweets in your timeline. For reference, in the timeline, Tweets with an image or poll usually take up more space than a 190 character Tweet.As such, Twitter is giving almost everyone 280 characters because this will give people more freedom and ease of use to express themselves. However, users of Japanese, Korean, and Chinese will not be getting the upgrade. Twitter argues cramming is not an issue in these languages because they're much more dense.
Some more stats can be read at the Twitter blog.