Great question! Yes, technically it is, but the correct grammar for it is "social medium", since "media" is the plural of "medium". This is likely the reason why Medium was thus named.

But the big difference with Medium is that it isn't funded by advertising. It doesn't have an incentive to keep us addicted so it can earn more money by showing us more adds. Its only priority is to provide us with content we like, and a platform to write our own content, so we keep paying our monthly $5 fee.

Facebook and Twitter would be much better platforms if they were funded by monthly memberships rather than advertising.