Facebook is the reason. Everyone you know is on Facebook. You could check Facebook hourly and find updates from people you actually know. If your friends are all going to a seedy bar, you’ll go as well – it’s the people, not the place, that make somewhere worth going to.
Google+ could be the greatest environment ever, and if your friends aren’t on it – why would you be?
That’s what all your friends are asking themselves as well.
Out of 350+ Facebook friends (most of whom I know in real life, and have a personal interest in those people), roughly 6 are on Google+, and of those 6, just one of them posts regularly. And I’m positive that the reason is the above. Why would you post in a place your friends aren’t, when you could post in a place your friends are?