Never took off? I thought it had hundreds of millions of users. In what world is that not taking off?
Where people actually *use* it. Sure, it has millions of users, precisely because google tied it to it's various services.
But do people actually post on it? Do people actually open it and use it to keep in touch with friends and family? No, and by your own admission, they couldn't because it gets "pretty heavy" when you keep it open.
Fanboy's aside, g+ isn't used and at least part of that blame has to be on the interface.