Let's say they have 12M monthly active users; a large percentage of those are probably already G+ users (let's call it 50%.) A large percentage of the remainder have deliberately decided they don't want to use G+, so you'd be doing pretty damn well to convert 10-20% of them over.
So that's only 600K-1.2M MAU in a "best case" outcome. You're just not going to be able to sell that as being worth allocating a half-dozen engineers to pursue building a new product. It's not even close.