I only see posts from people that I follow. There's a top content, or trending feature that Google+ has, but you can turn that off. I would suggest doing that, or perhaps following fewer people. Whenever one of my posts gets muted, Google+ tells me, but doesn't tell me who did the flagging. It is sort of disconcerting, so I try to avoid doing too much muting.
I don't think Google+ needs to use machine learning in Google+. I wouldn't consider "suggested people to follow" as machine learning, as it only suggests people that my existing friends (people I have in circles) follow. Of course, it is possible that Google+ is so subtle and powerful in its machine learning algorithms that it has inferred my preferences and adapted accordingly! If so, I don't think we are likely to find the details.
There is a question about the methods used for Google Predictions on both StackOverflow and SE Cross-validated, neither of which really could be answered with certainty. I think I answered the one on SO, which is a list of what Google Predictions does NOT use, as that is all anyone seems to be able figure out!