This is perhaps simplistic and applies mostly to this particular list, but volume has been actively discouraged in various ways over the last year or more by complaints and discussions on ways to decrease volume. Most often cited reason for complaining about posts was the quality of posts made by high volume posters - many who complained about that phenomena also mentioned that they were unsubscribing, or no longer actively reading the list as a result.
The point is that not all declines result from decreased interest or reflect a general decline in participation. As others have pointed out, traffic moves between lists - and more and more often, from public to private lists and other private forms of communication. One other interesting tidbit, though, is that conversation on IRC channels in English seems to have declined significantly as well. No idea how you could find or parse statistics for that.
Nathan