According to one study, Google+ is now the second biggest social network on the planet, overtaking Twitter and YouTube for the second spot.
There are some inconsistencies with the numbers, not to mention classifying YouTube as a social network, but the report does serve to highlight one thing, Google+ is possibly already bigger than Twitter and if it's not, it's only a matter of time.
Granted, everyone is focusing on the fight between Facebook and Google+, but it may be missing the point. Google may like to overtake Facebook, but that's not going to happen any time soon, maybe never.
In fact, while Google+ borrows quite a few features from Facebook, it also has a lot in common with Twitter. From the get go, Twitter was probably the bigger target, or, put another way, Google saw Google+ as fitting between Twitter and Facebook.
Whatever the case, the strategy is working, helped by Google constantly pushing Google+ onto its existing users, including YouTube users, which is another reason why counting YouTube separately doesn't make sense.