It was recently observed ( https://en.wikipedia.org/w/index.php?title=Wikipedia_talk:WikiProject_Medicine&oldid=582158481#Pageviews ) that total pageviews for some large projects on enwiki have been dropping for several months.
The table on https://en.wikipedia.org/wiki/Wikipedia:WikiProject_Medicine/Popular_pages shows that even though the number of articles in the project has increased, the number of pageviews dropped from 231M in October 2012 to 171M in October 2013.
To test this, I wrote a script that loaded a non-existent page (so that there would be no other hits to that page) every 10 seconds, reporting the number of hits every hour. Then I compared that to the number in the raw pagecounts files.
The full details are at https://en.wikipedia.org/wiki/User:Mr.Z-man/pageview#Test_4
Long story short, the problem seems to track the overall load on the site, with busy hours losing 20-30% and quiet hours 5-10%, around 15% on average. This suggests that the logging program is not adequately handling the load.
This may be the same problem as bug 49266