I think one reason why G+ never caught on was the awful discoverability. Every day I see a list of "People you may know" and 95% of those do not have any kind of bio or have anything publicly shared. How am I supposed to know if they are interesting? The only thing I have to go on is that we normally have one person in common in our circles. That could mean that the person is in the same field and potentially interesting, but could also mean that he/she is an uncle, old friend or something else that I don't care about.
Google should have forced people to have better bios or at least let everyone see a bit more about what kind of "sharers" they are to make it easier to grow the social graphs.
It could have done things better than FB, such as less advertising, less sneakiness and always show the entire feed, not some "top posts". But it basically did none of those and went with a worse UI, bad discoverability, bad spam problem etc.