According to the "Authors" graph at http://korma.wmflabs.org/browser/scm.html, in 12 months we have lost about 40% of code contributors (users that got their code merged in Wikimedia hosted repositories).
It is a significant number and according to the graph it's not an anti-spike but a consolidated number.
January 2014: 414
May 2014: 382
August 2014: 371
November 2014: 228 !!!
May 2015: 225
@qgil is happy to highlight this number to WMF management and our community, but first we would need to be sure that these numbers are correct and not the result of a software biug or another type of misunderstanding i.e. single users committing from multiple addresses before, and now only from one. CCing here different people that are good at looking at raw data and stats. Your help is welcome!
The Engineering Community team review is on July 6 (draft materials to be presented on June 30), and this would be a good context to bring these numbers.