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.
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.
Neat, I actually sold a wiki to Curse pre-Gamepedia and worked there for a short while. Not very happy to see a single company gobble up so much of the online gaming community.
It is unclear to ne if you refer to statistics or gut feeling here. Would you mind clarifying?