And you are considered to "use a platform" if you have created an account for it, and that account is still active (meaning that you can log back in if you wanted.)
I feel like this is an un-usefully overbroad definition of "use a platform", I think you should have to have at least actively used it in say, the last six months. Like, I have a Facebook account, have I posted on Facebook in the last five years? No. Have even looked at Facebook in the last six months? No.
This definition is kind of more "Are you the kind of person who goes around deleting accounts you're not using" than "do you actively use a platform".
Like, I have accounts on most things. I almost never delete accounts - who the hell does? That's a weird kind of specific behaviour that like, maybe 5-10% of people engage in. That's not a criticism of those people, but that's like, not "how people act" in general. It's the exception not the norm.
If we narrowed it even further to more than just reading posts, and pressing like, then it'd basically just be message boards, WhatsApp, Discord and Reddit for me. Like Twitch, I don't watch Twitch unless someone I know has specifically pointed me to a specific stream, have never booted it up without a specific goal, but I have a Twitch account.
Also is WhatsApp really "social media"? I guess it could be if you went around joining groups of mostly-strangers, and I guess some people do (and I get to hear horrible stories about such groups lol), but I literally only use to talk to friends and relatives (and occasionally co-workers, but usually only when they cross over into the friend zone).