Recently I saw this article that claimed Google+ has 150 million active users. (http://bit.ly/u9Mh3r) That's a bit silly. I can tell you from experience with large scale audiences (over @ MySpace) that it's nowhere near that number in terms of "activity" defined by any normal definition of "active" (sometimes companies try to say an "active" user is one that logged in once within a month, sometimes once a day, etc). Whatever the actual number is, it doesn't really matter because Google has a history of "winning" and here's how they do it: speed, or, you might want to call it efficiency.
Google won search not because its results were just "better," but because Google was "faster" (note: that's just a specific way of saying better). I don't mean Google search just returns results faster (though it does) and that the type-ahead is faster (it is): what I mean is that Google gets you what you want faster.
Google is gradually winning mail because Gmail loads faster than nasty Hotmail and so-so Yahoo. The clunky Web 2.0 or 2.5 interfaces those two dinosaurs have created in the last few years have only made them slower.. wow, bad move Yahoo/Microsoft!. Gmail also has better search, which helps you find what you want more quickly. There's a lot of confusing things in Gmail and it succeeds despite its nerdy features because it's faster.
Chrome is gradually surpassing IE and Firefox. Why? ... duh. It's faster. Once you use Chrome for a day and get over the fact that you don't know where to click, you simply can't return to IE or even Firefox. Why? Because they're slower and you value your time, because it's really it's your most limited resource.
Android is going to "win" phones because it's getting faster with every update. It loads faster on more powerful phones and the interface gets you to what you want more quickly (note: Android has as already surpassed iPhone for total installs, but it'll win the "App" / "paying App user" market as well). Speed is the main reason I use Android over iPhone (and the fact that the menu, back, home and search are always in the same place -- those App developers on iPhone with their own place for every menu button will drive you crazy). (Note to Google: removing "hard" buttons on Galaxy is a mistake!)
I could go on, but you get the point... G+ is going to continue to grow and grow as long as Google does what they always do: make things faster. If it's quicker to follow someone, create circles, distribute and consume info on G+ then you'll do it on G+. Facebook's new timeline is not faster. It's confusing, and thus slows you down. Some will love it (like people liked crazy MySpace pages), but efficiency will win in the long run. G+ is like the little engine that could. It'll keep tugging along as every product & aspect of Google is attached to the engine (http://bit.ly/mUzjb4) and G+ will keep moving forward . Since it's Google, the cars being pulled by the train won't slow it down -- it'll get faster & faster until it's the fastest way to do just about everything online. (You can start to see some of these efficiencies in the integration between Android, Gmail, Youtube and G+.) All this is a game changer for Google as company. Very exciting to watch :-)