Why Are (Some) Tweets Getting Shorter?

I like Twitter and I like scientific papers. So I like this new paper by University of the Philippines researchers Christian M. Alis and May T. Lim: Spatiotemporal variation of conversational utterances on Twitter Using Twitter's API, they downloaded 229 million 'conversational' tweets from 2009-2012. They defined as 'conversational' any tweet starting with the character @. These are messages directed at one or more specific users, although anyone can read them. The headline finding was that @ tweets are steadily getting shorter, as can be seen in the graph on the right showing linear downward trends over the three quartiles of the distribution: