Measuring the quality of communication by the number of emails is like measuring the quality of code by its number of lines. And I guess you know the comparison to aircrafts :-)
I know first hand what happened to the perl6-language mailing listPerl 6 matured, and the mailing list didn't. I asked about how some few methods are named, and some of the replies suggested to rework the complete object system. So the discussion moved away from the mailing list (to IRC, blogs, nopastes, the bug tracker of the perl6/specs repo on github, real-live conversations and so on).
So to me, the decreased number of emails on this mailing list indicates a shift in communication, not a decline.
I also have the general impression that some of the mailing list discussions have moved to blogs and twitter.