Currently tracking manually Flow activation (T114100: Track Flow requests from Wikimania 2015 to the end of September 2015) based on https://www.mediawiki.org/w/index.php?title=Flow/Request_Flow_on_a_page
With the opt-in option (T98270), we will need to know how many users have activate flow per wikis.
See also: T63747: Flow should be able to enumerate all flow enabled pages on a wiki