So by that metric, if every gmail user used G+ it would still be a failure? If it captured 70% of the online population, it wouldn't be that impressive because those people originally used search?

Wave was a failure, and it had plenty of Docs and Gmail users to draw from. G+ strikes me as a very different beast, and one that has already found its niche. It may not displace facebook, but it certainly seems to have eaten into Twitter's niche.