Although I saw the original tweet, I didn't really think about the observation until you posted it here John. Perhaps that says something about blogging, although not sure what.
However, I'm also not sure about Frances' observation. Whilst a blog post might have greater longitudinal visibility due to the way blogs work (compared with tweets), does it necessarily follow that it has a greater impact, at least as measured by comments? Might it be possible that a tweet 'touches' more people (with RTs etc), albeit for a shorter time period?

I think you ought to start a PhD to find out ;-)