My personal criteria to specifically identify social media apart from other online interaction:

* The platform is a closed garden with a goal to own the content, user submissions, and any personally identifiable data or relationships thereof.

* User interaction is primarily limited by a terms and conditions policy as opposed to a code of conduct. The goal is to impose constraints upon user rights as opposed to user behavior.

* There exists a profit incentive directly tied to engagement frequency. The goal is to quantify content visibility and sell those numbers to third parties.

* Exchange and reselling of user profile data, user submissions, and any analysis or relationship there upon is beyond user control, awareness, or agreement.

What about Mastodon or other federated platforms? If someone created a new Facebook, but it didn't have these corporate terms, it would still be social media.