Updated usage statistics are now available at http://jenkins-ci.org/census This data has been scrubbed of distinguishing information as much as possible, filtered for installations we've seen at least twice a month with at least one job set up, and split up into monthly JSON files. Starting with August, we only have data on installations of version 1.368 or later - before that, reports go to the old Oracle-owned server, and we aren't able to retrieve them. So that's why you'll see a fairly hefty drop in install count, etc from July to August.
I haven't had a chance to do any analysis to speak of on the data. I intend to get the plugin usage spreadsheet updated n the next few days, but I wanted to make this data available to you all ASAP. If you come up with any interesting analysis or use of the data, please let us know!