Well, thats the honest and straightforward way of calculating, but from a competitor standpoint, you wouldn't want to match feature to feature and then try to compete. You'd attack by taking over the disgruntled users first, and Google has done that well with Circles, and Privacy. Google has its own issues with acting like a drunken sailer kicking off people at whim. But I doubt it'd take 2 years - if G+ managers to build engagement and retention, they are good to go - but honestly, thats a battle for Google.