I really don't think they are in the second place. Twitter fills that spot.
The thing is, they had a chance to capture the market of early adopters, but they've lost it in the nymwars.
And I know a lot of people try to come up with all sorts of reasons for why Google+ didn't stand a chance against Facebook, ranging from UI to availability of games and the network effect, but no, Google+ appeared at a moment in time when people where nervous about their online privacy and getting tired of Facebook and Google had one chance and totally blew it with their real name policy.
But then it got worse, they forced YouTube users to migrate to a Google+ account and thus adopt the real name policy. I mean YouTube, their biggest online community. It has been a total clusterfuck and guess what, YouTube comments are still shitty, even with real names, because many people are in essence jerks and a real name policy isn't going to change that. I sometimes wonder if they were clueless about this, or knew it and did it anyway for advertising value.
But lets rewind a little and remember that they killed Google Reader because of Google+. I'll never forget them for Google Reader, I'll never trust their free offerings again.
If anything, their current attitude of backtracking from their Google+ push users-be-damned has been nothing but healthy.