I think there's a distinction between systems where users follow people and systems where they follow topics, I think of the former as social media, not the latter.

Twitter, Instagram - social media. Reddit, Stack Overflow, HN - Not social media.

StackOverflow? LinkedIn? Is any website with user accounts "social media"?

Here is a definition from dictionary.com

"websites and applications that enable users to create and share content or to participate in social networking."

So having a user account doesn't make it social media.

Stackoverflow seems like a "no" but you can definitely promote yourself and there is a "community" there.

LinkedIn is virtually all about social networking, so yes.

And Wikipedia? And email? AOL? Compuserve? Citadel BBS? Finger?

dictionary.com defines social media as "websites and applications that enable users to create and share content or to participate in social networking."

So the common use case of wikipedia, reading about things, is not. But digging deeper into the editors\editing section there is definitely social media happening both in forming networks and creating content for others

I don't think email qualifies. While you can share content and network with it, it seems to be more of a delivery mechanism than a destination that users would seek out.

"AOL" is too ambiguous.

I always though of Compuserve as a service provider but maybe they did have some social media features

BBS might be the original social media

Finger is interesting as you could create a "profile". myspace and other social media seem to be a combination of finger and bbs packaged into an endpoint

I've always thought the key characteristics of social media is sharing to a social network. When I post here or on Reddit I have no idea who it's going to.

Facebook and Twitter are social networks because I share stuff and there's a definite social graph that I'm part of.