It is unclear to ne if you refer to statistics or gut feeling here. Would you mind clarifying?
- Wikimedia
- FANDOM
- Gamepedia
- Miraheze
FANDOM is the most massive wiki farm with over 360,000+ (as of 2016) wikis[1], which I'd give at lowest an estimate of 60% of the total number of wikis on the net, and is 88th on the Alexa rankings.[2] FANDOM is a wiki powerhouse, and you bet it uses MediaWiki.
Excluding WikiHow, I have never seen a wiki not use MediaWiki. As one of the guys that hops across many different wikis and wiki farms doing automated work, I cannot stress this enough.
[1] Brandon Rhea, FANDOM VP of Growth https://community.fandom.com/wiki/Choosing_Fandom?diff=next&... (dated June 14, 2016)
[2] Alexa.com https://www.alexa.com/siteinfo/fandom.com#section_traffic (dated ~21 July 2020)
WikiHow is using mediawiki (or at least a fork of it) https://src.wikihow.com/
But there certainly exist other wikis and wiki-like projects that dont. MDN and OWASP wiki are prominent examples that moved away from mediawiki. I think mediawiki has most of the mass-collabotation market, but there is much more competition in the open-source project documentation niche (which people often use wikis for) and corporate knowledge base market.
P.s. for the interested, mediawiki has statistics at https://pingback.wmflabs.org/#unique-wiki-count (opt-in) and https://wikiapiary.com/wiki/Main_Page (based on web crawling)
