Google+ has split opinion and had its fair share of detractors, most notably around forcing Google+ accounts on its users and the Youtube comments saga. It did have a lot of good things going for it too though, most notably Photos, private location sharing and Hangouts, but it never became the force Facebook or Twitter were. Since Vic Gundotra left the helm it seemed Google was giving Plus a back seat, focussing on some of the best parts and making their own apps out of them or rolling them into things like Maps.
Google has since seemed to shift Plus from a social network to a place to share content with like-minded people, focussing on Communities and Collections making it, in my opinion, more of a Reddit or a Pintrest than a Facebook.
This focus has been taken further with a redesign that you can opt in to via the instructions here Dive into the new Google+
Update: The new layout wasn't appearing to me so I followed these instructions on android police and it worked: "You can try out the new Google+ now by going to Settings > Manage other apps & activity > Manage Google+ activity and clicking in the search bar at the top of the screen".