That number is inflated by adding YouTube users, which was a kind of social network with it's own kind of audience (notoriously trollish in some ways) before the shotgun wedding to G+.

My follower growth has slowed. The first product page I put on Google+ (a book about Android) got many followers quickly. Same with my personal page. The second time I put up a product page for a book, the response was much attenuated, and my follower growth has been slow and low quality. It wasn't great quality on Facebook either, but it was much faster. Maybe I need to see how well product pages would do on LinkedIn.