Ripard Teg put together several handy little graph consisting of the average concurrent users in EVE in 2015 and posted a link to them to Reddit. I posted one covering the last 18 months below.
When I look at the graph, I think of two things. First, that null sec is stagnant and a lot of players are waiting on the implementation of the new null sec sovereignty mechanics next Tuesday. I'll go one step further and state that the delays in implementing the new system aided in the decline in the average concurrent user numbers. At the beginning of March, CCP announced that the sov system would appear on Tranquility in the June release, or 2 June. But on 7 May, a dev blog announced that the actual capture mechanics were delayed until 7 July. Shortly after that date, the graph started to edge down. The trickle grew into the beginnings of a failure cascade on 1 June when CCP announced another week's delay, until 14 July.
With the null sec powers playing a waiting game, one group became null sec's punching bag: Brave Newbies and HERO Coalition. I don't know what BNI's participation rate was at the end of the fighting in Fountain, but I hear the numbers were poor. I wonder how much of the recent decline was a result of members of Brave just refusing to log in and engage in another fight they probably could not win.
I could go into an analysis of why a drop in activity is normal for a 13-year-old game. But FozzieSov is dropping in 6 days. Perhaps CCP will amaze and confound conventional wisdom one last time.