A few days ago, I introduced a spreadsheet listing Twitter accounts for the 50 largest transit agencies in the US.
I've now updated the spreadsheet with links to agencies' homepages, blogs, Facebook pages, YouTube channels, and Flickr streams, where they exist.
You may find this summary to be helpful. If you're keeping score, Twitter is still the most-used of the social media sites.
Finally, a brief note on methodology: I didn't look that hard to find an agency's blog or social media accounts. If there wasn't a link on the agency's homepage or a page that was one link deep (at most), then I gave up (as I expect most riders would). I did not search on the social media sites themselves in order to avoid spoofs, parodies, and other unofficial sites which are rampant on Twitter and such. So, in order to be considered, an account had to be linked from the agency's Web site.
Now if only these data were actually captured in the NTD...