Via the Lindau blog:
The verdict: is blogging or tweeting about research papers worth it?, by Melissa Terras: Eager to find out what impact blogging and social media could have on the dissemination of her work, Melissa Terras took all of her academic research, including papers that have been available online for years, to the web and found that her audience responded with a huge leap in interest...
Just one quick note. This is what happened when one person started promoting her research through social media. If everyone does it, and there is much more competition for eyeballs, the results might differ.
We are a long way from "everyone" doing it (ahem), so my guess is that there are still gains to blogging about research. Take a look at this paper to see some empirical evidence: The impact of economics blogs (McKenzie and Ozler).