Well yes, if the users don't want the product then nothing else matters, but I don't see what that has to do with innovation.
I have no problem with your argument that Google+ will need to be useful in some sense to be successful, but originality is hardly necessary for that to be the case.
If what they're going for is "Like Facebook, but better!" then they certainly are going to have a difficult time convincing many users to switch, but I wouldn't say it's anything approaching impossible.
Not if it's something people don't need or want in the first place.
If Google have 'innovated' then it means they've created something original and new that is useful for people. Only time will tell, but my personal opinion, based on what I've seen so far is they haven't.