In my view a platform is something on which you can build anything. TiddlyWiki is a software platform. I am the owner always.
TiddlyWiki can be also made available on many different
server platforms, you choose.
Facebook is not a platform although it has its own server platform, we have little or no control how content we place on it looks and appears to others. They give us a network but control its operation to extract advertising revenue.
Ultimately this is a matter of semantics. But I see mediawiki as a wiki platform you can place on a server platform. Wikipedia is specific instance on a server platform.
Regards