Stupid graph, that's comparing apples and oranges. Yes products are identical, but the distribution is totally different. Facebook intentionally was only on college campuses. It let them scale slowly and work their way up the chain so to speak. Twitter was crushed by growth and didn't have an existing user base to tap into. Google+ growth is impressive, but it's built on top of how many Google Accounts, GMail account…
OK, yes, apples and oranges, but it is not a "stupid" graph. The graph provocatively illustrates that google+ could steamroll over facebook in a very short time, if they play their game right.
Google+ is making a huge splash, that is about the only meaningful statement that this graph illustrates. It doesn't suggest that they could dethrone facebook because there's a hell of a lot more to making a good social network than how quickly it gets ten million users.
For me and my friends, google+ is a dead zone. I signed up to check it out, and barely have looked at it since. I would like to use it some day, when more of the people I really want to connect with are on it, but there's really no certainty that it will make it to that point.
Until then, be very wary of false extrapolations.