Uh, they kinda did copy Facebook shamelessly on the core features and basic UI, with the exception of Circles. I log in, I see a feed, I have a way to add “friends”, I can share text/photos/videos/links and comment. It basically looked and felt like logging in to Facebook with no friends on there, less features, and a few interesting UI differences.
If they had done something different, like say restricted you to X number of friends, for any small X, or focused on groups, or done something anonymous like Secret - now that at least would have been different. It may have still failed but at least it would have been trying to scratch a different itch on the social spectrum.
I think the parent meant that Facebook had more features so Google+ didn't really aim at "feature parity".
If it had copied Facebook shamelessly, it would've given an alternative for all those people fed up with Facebook for the last decade or so, and thus increased usage too.