For example, boost-users has a posting rate nearly a third that of before 2011.
Regarding this, my personal (weakly substantiated) perception is that people are migrating to sexier forums such as stackoverflow to get the same help. For instance, 5-6 years back I could get about 1 question a week about Boost.MultiIndex on the users list. Now it's like 1 question every 3 months here, but 1-2 a week in stackoverflow instead. Blame the crude interface a mailing list provides, registration entry barriers etc.
As for the decline in the dev list, it looks more serious and there's no migration theory that could mitigate it. Also, I feel like the maturity level of discussions has fallen :-(
Joaquín M López Muñoz Telefónica
Frankly I don't see this that negative, at least not yet. Maybe the decrease of volume is caused by the fact that libraries most people were using are now part of the STL? I consider this might change as soon as the mental barrier we're having about writing C++11/14 only libraries goes and new cool libraries are written using the new language features. I'll try to do just this next week at CppNow. Maybe we also need new people to shake the tree and take an active part to replace those who are now less active. Cheers, Christophe