Earlier today, for reasons I'll leave out, I found myself wondering how much less use of LJ there is in Iowa than in California, which led me to the raw livejournal stats page. On the way, I noticed that Massachusetts is the #6 state by number of LJ accounts, even though the rest of the top 10 have significantly higher populations. So I pulled up census.gov and divided some numbers...
Approximately 4.8% of residents of Massachusetts have LiveJournals
For California, it's about 2.5%. Iowa, 1.4%, and Idaho, 1.0%.
I think MA has the highest LJ percentage of any state, and perhaps any country. Almost 1 out of 20!
... but I didn't have time to check any more states, or write a script, so I'm not sure :)