MediaWiki uses MySQL for primary storage.
The IndieWebCamp community uses MediaWiki as a collaborative document editing tool, to capture "what is the current state of something", to make it easier to reference answers, definitions, how tos, etc.
MediaWiki just happens to have better persistence / search discovery than alternatives that are equivalently community accessible (E.g. Etherpad, Google Docs, both of which have the js;dr problem and neither of which are search engine indexed.)
Other Independent Examples
Friend of GWG
David Shanske has setup MediaWiki for a friend as a private thing and maintains it for them.