But still plenty popular - at it's peak it had 200 million users. For comparison, that's nearly 2/3 the population of the USA

That metric is very misleading.

That is the number of Google accounts that didn’t go through the effort of disconnecting from / opting out of Google+. For a few years, all new accounts were automatically enrolled in Google+. It’s likely an extremely inflated number compared to the number of users that engaged with Google+ social features.

Citation: https://arstechnica.com/gadgets/2012/01/google-doubles-plus-...

I agree with the sibling that that isn't a representative number. I think we'd want to know something related to actual participation - what's the average daily or weekly number of posts or likes or whatever equivalent is in Plus versus Facebook, Twitter, etc.