Sure, getting to fifty million users within 3 months is a pretty impressive stat.
But isn’t it fairly easy to get to any arbitary number of users if you already have a set of well used products with a large user base like GMail and Reader. Especially when Google are directly integrating Plus into them via the new toolbar.
A better measure might be how many incremental customers Google have gained thanks to Plus, and then how many of them are using the service daily.
The biggest challenge to Google plus’ growth is the number of social networks which their target audience are already engaged with. While Linked In and others appear to have attempted to directly mimic Facebook, those services at least have enough different use cases from Facebook for people to use both. Perhaps Google need to focus more on their key differentiators, such as “hangouts” in a bid to attract new users away from Facebook.
Maybe then that will attract users from the outside in and Google Plus will get its mojo back?