What you're seeing is a subset of 229 million tweets gathered from four years of Twitter usage. The researchers tallied up the number of words in each tweet and correlated them by the year they were published. The chart above represents all the tweets that were sampled on the first Friday in December of a given year. Over time, the results show an initial increase in the average tweet length, followed by a rapid decline.
In 2009, the share of tweets numbering 40 characters or fewer was pretty balanced against the proportion of tweets numbering 120 characters or more. The following year saw a spike in the proportion of tweets numbering around 120 characters. But ever since then, tweets have been getting shorter and shorter — to the point that long tweets now account for only a tiny fraction of all tweets.