if you look back a few more years you'll see that is the trend, at least since 2013. whether it is fewer people posting in the thread, or an actual decline, i'm not sure i could say. it does seem like many people forget to post, and then at the end of the year add their total then; some people seem to continue adding to new year beer to the old thread as well, though, so i'm not sure which number is more accurate. at the end of the 2013 thread, @JR_Brewer made a plot of homebrew produced vs month of that year, and december is heavily skewed due to people catching up with the thread. with that, here are the 12/31 midnight stats for the last few years and the end of the thread totals.
2016: 23,767.5; 25,146.5
2015: 30,885.5; 32,125
2014: 47,189.5; 48,576
2013: 59,796.5; 61,758
2012: 38,764.5; 41,162.75
2011: 48,010.75; 50,374.3
2010: 40,957.5; 42,539
2009: 28,710; 30,862
if there are statistics that show site activity as a function of time we could determine if this is a problem of people posting or actual amounts of home brewing.
maybe its a good idea to make a new thread to discuss this further so as not to jack this thread.
*edit: noticed the number format wasn't consistent, so i fixed it