By 'MediaWiki' I'm referring to the wikis and wiki farms that use MediaWiki or a variant of it. This includes:
- 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)