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

They are all media that you interact with and also interact with others via. The distinction is made with television/radio, where interaction is purely one-way, and is therefore not social media.