Remember: Facebook was developed in a world with existing and mature social networks. What gave FB a marked edge wasn't its openness but the fact that it was exclusive: Harvard, Ivies, selective edus, all edus, general world.
For the first few years of its life, Facebook was the place that outiders viewed as more selective and attractive than any other option. This was a powerful growth hack.
My own feeling was that Google had a solid initial nucleating community, mostly technical types but also some creatives (largely designers). Unfortunately, by opening the floodgates early, one of the larger adopter communities were SEO and marketing types -- not, shall we say, the most engaging party animals. The turn came early.
I also feel that G+ really didn't start getting its feet under it until ~February/March 2013. Search had been incorporated, there was a growing wealth of content that could be found. I made several posts aimed largely at G+ staff about the value of search-driven and directed navigation rather than streams (I've deleted those along with most of my content).
Unfortunately subsequent changes (the May, 2013 UI redesign) negated many positive effects. One telling post a month or so later from Circlescope showed a marked drop in engagement following that release. I continued to fight for it but watching increasingly passive-aggressive demands for my personal information (phone number, integrated YouTube account) were just pumping up the creep-out factor.
Overall I do agree with you on UI/UX -- G+ is pretty awful in that regard.
These minor grievances could hardly be more narrowsighted and petty in that context. I do not believe in the 'make or break' theory you work against.
It took far too long to get an invitation, and you had to be 18 or older to use it. Facebook exploded first by being used by kids still in Elementary and High School. They initially cut out an absolutely major demographic, and people lost interest after a couple days.
Last I checked, Havard University, Stanford University, and the Ivy League are neither elementary nor high school institutions.
Strike Two: Combine the "Real Names Policy" and forcefeed G+ to youtuber's who want to comment. No body saw the obvious problem with that? Some may not want a LOL on a Cat video be one search away from their real name. Plus (!) it was rolled out badly - some people began to get locked out of Gmail over this.
Strike Three: Why is the UI so complicated to integrate Hangouts? Who does UI design? I still get Hangout Notifications from two previous companies.