In demographics we can see pretty good information about all data sources community. Mailing list members are going down also (mailing lists are used less? where is the support of the projects moving on?),
Data about mailing lists, IRC and even wikis is good to have but I'm not
sure they are key. These channels support the real deal: code repostories (and Bugzilla too). For instance, a % of discussions that happen now on Gerrit probably took place before at wikitech-l.
I find that hard to believe (unless you are going back to the pre Special:CodeReview era.) Perhaps there is an increase of people talking face to face/internal wmf meetings over using wikitech-l. Perhaps the (possibly imagined) increase in bikeshedding is making people less likely to post to the list.
-bawolff