It's just an open protocol for publishing and reading optionally-structured data, with extremely light social networking fundamentals baked in (follows, etc.). "email" was maybe a stretch.
"Social media" as a definition presumes "cOnTenT" - and delivered and organized in a particular way.
Look at the hundreds of "Other Stuff" apps. "bounty marketplace", "raw milk vendors", "digital pet app", "private legal systems", "cryptographic signing", "payment splits", etc - if these are all "[social] Media" then the term is so broad as to be meaningless.