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.
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.
Facebook and Twitter are extremely popular services, and have been at or near the top of their categories for over a decade.
Google+ was an attempt to challenge them, shut down after it failed without ever becoming anywhere near as popular.