G+ positioned itself against Facebook sort of like DuckDuckGo went against Google: we made the same product, but fixed X!!!, where X is some gripe about the incumbent's product that only a small percentage of the product's potential userbase cares about (privacy, in both of these cases).
That was (and is!) a fantastic strategy for DDG, for whom a fraction of a percent of all search traffic counts as massive, life-changing success. Google is not DDG, though. G+ would have needed a much larger share of the social networking market to be considered a win for Google, and the initial differentiation was not anywhere near clear enough to get there against a rival as strong as Facebook.
I also agree about the UI mess, and all that, of course, it was not a great product to use out of the gate.
Conceptually they had the core of a very good story, which was the idea I could have "Facebook where I segregated the people in my lives". It was even quite good timing, as it was mooted around the time people were starting to bubble up stories about "my boss started reading my Facebook and I got fired", "my family are upset by the politics my friends discuss" and so on.
The biggest problems with G+ from my perspective, around launch time, was other than Facebook's network effect, that the dribbling out of invites completely cut against the core success model for a social network, and the whole circles functionality ended up being one of those UI nightmares you get from Googlers who don't understand how normal humans actually work.