> Why do you consider TikTok to be social media

A reasonable definition of social media is that it refers to the means of interactions among people in which they create, share, and/or exchange information and ideas in virtual communities and networks. "

In other words, roughly: a medium in which media created and consumed by the crowds vs. institutions.

Also see https://en.wikipedia.org/wiki/Social_media for a longer description.

> Is YouTube also social media?

Yes. "YouTube is an American online video sharing and social media platform headquartered in San Bruno, California." -- https://en.wikipedia.org/wiki/YouTube