"Who uses APL in production?" is a common question. These days, it's mostly those who haven't stopped using it. The historical use, while never quite mainstream, might be surprising to those who never realized APL had a commercial presence at all!

- APL conferences were held every year from 1969 to 2004, minus '77 and '78 due to some unfortunate logistical issues (certainly not lack of demand). Conferences in the 80s regularly had >1000 in attendance despite both IPSA and STSC holding their own many years. My count (may not entirely rule out duplicates) is >1000 paper authors, and far more papers were submitted than accepted. https://aplwiki.com/wiki/APL_conference

- At least a dozen hardware manufacturers implemented an APL for their systems in the mainframe and minicomputer era (70s and 80s): https://aplwiki.com/wiki/Vendors

- Usage stats from just one university in 1978: 7,300 active accounts and 5,400 workspaces (which would each store an individual project, or something like that). Used for all sorts of administration and even a student ride sharing service. Today of course, much of this would be done in Excel. https://aplwiki.com/wiki/Syracuse_University

- The first serious e-mail system was implemented in APL in 1972, and used by Jimmy Carter in 1976 for his presidential campaign: https://forums.dyalog.com/viewtopic.php?f=30&t=1629

- I've heard but can't verify that one of the APL books (Gilman and Rose maybe) sold over 100,000 copies.