Forgive me but doesn't that mean they were actually using google+ in the first place? Which seems.... unlikely? Or at least incredible outliers.
You won't get the majority of people on Facebook to pay for the service after you've been giving it to them for "free" for 10 years, you'd maybe convert 10% if that, at which point what use is facebook with most of the users gone?
It's difficult to ask for users to pay for a service that was always free, indeed, furthermore facebook is not growing its user base anymore.
But a competitor can emerge by providing an alternative service, for a fee, marketing it as "the service that doesn't spy on you".
Now, sure, that will be a niche service, and social networks can't be niche services. The protonmail of facebook or twitter will probably never emerge.
We were all using google+ to share images. Some of them are on facebook (my mother and sister in particular), but the rest of my extended family is not there. But we were all on G+, I told them if they wanted to see baby pictures you have to sign up.
I couldn't care less about anyone outside my family, if they want to contact me they can email or call me by phone.
You won't get the majority of people on Facebook to pay for the service after you've been giving it to them for "free" for 10 years, you'd maybe convert 10% if that, at which point what use is facebook with most of the users gone?