Google+ is to Twitter as Usenet is to IRC. Discuss.
(I'm writing this down just so I don't have to think about it anymore; I don't know what Facebook is in this context.)
To elaborate: Google+ is like Usenet, except with no newsgroups, and most of the control over what you read being done through kill files and keep files. It's so busy that you can never catch up, and the default group is a combination of rec.pets.cats, talk.bizarre, and news.announce.newusers.
Twitter is like IRC, except that you're all on one channel. The hashtag syntax creates things similar to channels, which makes the whole mess easier to keep up with as long as your interests can be narrowed.
In both cases, the inversion from the original system is that the fundamental unit of following is no longer the topic, it's the person. If your favorite Dr. Who fan also publishes lots of photos of their cats, you have to read them both together. There's no way on Google+ to carve out a narrow channel for deep discussion, not yet at least – you would expect that is the natural evolution, just as Twitter added support for emergent hash tags after they had been invented.