Melissa Terras has published "The Impact of Social Media on the Dissemination of Research: Results of an Experiment" in the latest issue of the Journal of Digital Humanities.
Here's an excerpt:
From now on, I will definitely post anything I publish straight into our institutional repository, and blog and tweet it straight away. After all, the time it takes to undertake research, and write research papers, and see them through to publication is large; the time it takes to blog or tweet about them is negligible. .