I highly recommend the three-post series by David McKenzie and Berk Ozler on the impact of blogs:
The third of these posts wraps up,
There are large impacts on dissemination of research; significant benefits in terms of the bloggers becoming better known and more respected within the profession; positive spillover effects for the bloggers’ institutions; and some evidence from our experiment that they may influence attitudes and knowledge among their readers. Blogs potentially have many impacts, and we are only measuring some of them, but the evidence we have suggests economics blogs are playing an important role in the profession.
This is very encouraging, more so because this was a quantitative investigation. The results are consistent with my personal experience.