In Wired, Clive Thompson points out that a large study of writing and rhetoric concludes that students are not only writing more, thanks in part to the social web, but how they're writing differently.
[The study] team found that the students were remarkably adept at what rhetoricians call kairos—assessing their audience and adapting their tone and technique to best get their point across. The modern world of online writing, particularly in chat and on discussion threads, is conversational and public, which makes it closer to the Greek tradition of argument than the asynchronous letter and essay writing of 50 years ago.
...it's also becoming clear that online media are pushing literacy into cool directions. The brevity of texting and status updating teaches young people to deploy haiku-like concision. At the same time, the proliferation of new forms of online pop-cultural exegesis—from sprawling TV-show recaps to 15,000-word videogame walkthroughs—has given them a chance to write enormously long and complex pieces of prose, often while working collaboratively with others.