One class of statistic which, I think, readily paints the picture is the reputation of undisputedly popular posts. The Best Nodes over the past year have rep in the 50s to as high as 90. In the old days, breaking 100 was not unusual. In contrast, today's 50 Selected Best Nodes all have rep over 99; 5 are above 190, the highest being 260. Not one of those 50 is later than 2008. Of course, nodes which have been around since 'the old days' have had more time to collect upvotes; but I believe that in general the vast majority of votes are cast on a node within the first few weeks of its existence.
Another way to see something similar might be to look at the reputation of your own "best nodes". My 49 best nodes are all 2010 or earlier; and all of my nodes which are better than 90 rep (there are seven of them) are 2007 or earlier. No post of mine has hit 40 rep since 2012.
I am not a prolific poster, though; someone like BrowserUK might have a more meaningful trend line.
(Update: s/XP/rep/g for nodes.)
I reckon we are the only monastery ever to have a dungeon stuffed with 16,