For example, the number of subscribers in /r/DarkNetMarkets or the number of registered users in Agoras community forums. I think it would be really useful to see these stats.
On a side note, what would you say the growth of the various dark markets has been like these past six months?
according to our 'Traffic stats' page (only available to mods, sorry. we encourage posting at all hours) This sub has seen a 50-60% increase in uniques [are the total number of unique visitors (determined by a combination of their IP address and User Agent string) that generate the above pageviews. This is independent of whether or not they are logged in.] 90,000 to just under 150,000.
Since February, we have been hovering under 2,000,000 pageviews [are all hits to /r/DarkNetMarkets, including both listing pages and comment pages] a month.
The amount of subscribers daily is about 75-110, with a huge outlier every month or so. The last outlier was 474 in one day. The amount of unsubscribers is untracked.
We rarely drop below 1.5K pageviews per hour.
As far as tracking markets, I know /u/gwern does weekly spiders/"site scans" (cant think of name ATM), of most of the major sites, but this data is unreleased to my knowledge. Previous data about SR and Reloaded are available at his website, gwern.net