I'm not sure where the article gets their numbers, so I can't comment on that.
However, I will say that Google missed their real window in launching Google+. It seems Google just doesn't have the "knack" of advertising and seizing on opportunity, despite being an advertising company. Don't get me wrong, I like Google and all that. I just think they need to hire some PR folks, rather than letting the engineers run things.
IIRC, about a week after Google+ started in "invite-only beta", there was yet another security fiasco with Facebook. But this one was big, really huge. It was all over the news, it was all I heard about, and it seemed like everyone I knew was threatening to jump off Facebook because of it. I thought, "Someone at Google is watching this, and is going to open up the beta to everyone." But no one ever did.
That would have been the perfect time to really open up Google+, when everyone wanted an alternative to their ongoing security woes at Facebook. Never happened. Google continued their sloooooooooow rollout of the "invite-only" beta. Finally, months later, Google finally opened Google+ to everyone who wanted to join.
But it was too late. Google+ was a ghost town. Only a few people I knew were on Google+. The rest of my friends eventually "got over" whatever the Facebook security problem-of-the-day was, and stayed on Facebook. Since my friends are on Facebook, I stayed on Facebook.
For all that, Google+ does have a killer feature: Hangouts. I wish they'd made a big deal out of this when Google+ launched - like, showed it in action or something, ads on TV, whatever. It's like Skype or any other video chat, except you can have up to 10 people on at the same time (you + 9 others.) We have a hosted domain for work, and we use Hangouts all the time to talk to people at different locations within the organization. It's really freed us from having to share a single video conference room at our location.