It is not that the activity of voting on older posts is just being spread out on more posts. It is the activity for all posts together which has gone down to nearly zero.
Those curves are for the total number of votes. And not votes per post (both questions and answers).
We have a growing number/bulk of posts, but after a week (or even a day) nobody looks at it anymore (and those few old top posts are not making much up for it).
That is why I say that the activity on the website is dominated by ‘frontpage-activity’:
- helpers that answer very specific helpdesk type and homework type questions (which some see as detrimental because it encourages lower quality questions)
- and competitors that search for new/hot/active questions in order to score.
(I have done this as well; while questioning myself for low scoring answers and searching specifically for popular questions that give potentionally high scoring answers; and yes it works, you can game the system in that way and ‘score better’)
The ‘canonicizer’, ‘challenge seeker’ and (I am adding) ‘quality controller’ are getting less ground.
It did not use to be like that. There used to be activity that was about older posts, past activity, as well (at least the voting). See also the following graph which shows the distribution when questions (on average) acquire their score as a function of their age (how much do posts increase their score in the first month, in the second month, etc).
(above is a graph for StackOverflow; see here for a graph of electronics.se)
See also On the value of scores and votes - #4 by MartijnWeterings for typical growth curves of the score of posts (you see that those answers acquire a lot of their score in the first day; in the old days questions still kept on growing their score with a reasonable pace, or even get into a polynomial growth; but nowadays the answers and questions on average get very little extra score after the first day and the pace by which they improve their score is decreasing)
Here we see indeed a dilution effect. In the early years of SE/SO a question would on average score more and more as the age of the post increase (except an initial drop after a short high peak in the beginning). But now questions only get their score in the first month (you can zoom in further and it is actually almost all in the first day or even first hours).
Questions (and also answers) do not last as long anymore. They only get one day or less to grow up before they are thrown into the big pool of old questions. It is becoming a rarity when they get a (up)vote or two in the process.
(It is also a difference in the style of questions. It is much more difficult to make a question like ‘the old days’ which has more canonical type posts. But I believe that the scaling and aging of the site plays a strong role as well.)