I don’t think Google needs to do anything, other than keep the service maintained, eventually Facebook will finish crashing and burning itself out. A lot of people have already stopped using Facebook, or they keep it for Facebook exclusive people and connections. Most of the people who actually use Facebook are either senior citizens, teenagers or people addicted to Facebook games. That crowd will narrow down as the seniors want to try something new and less complicated. The teenagers will outgrow the social feed marathons. If that just leaves people addicted to Facebook games, that’s not enough to keep Facebook afloat as anything but a gambling den. (In reality, Facebook games serve no purpose and they are a lot like gambling addicts in the way they must collect web graphics and even spend money on getting more of them).
Anyway, it really seems to me that Google+ will be the winner of the struggle with Facebook, it will just take time.
Meanwhile, Twitter take heed. Don’t try to become more than you already are. Too many features will just make it complicated and ruin the simple function of a great idea.