Among everyone who visited Google's nascent social network over the last few months, the average amount of time they spent there during the entire month was three minutes. Yeah, you read that right - three minutes out of the entire month. By comparison, those who visited Facebook spent an average of 405 minutes per month, or nearly seven hours.
Of course, average numbers only tell part of the story. With a giant like Google, there are bound to be a huge amount of people who just pop in and pop out, but no matter how you slice it, three minutes is kind of pathetic. At this point, there is little reason to do any kind of social networking beyond Facebook, which is where all the content (social games, etc) is. Google+ does have some different features, like "Hangouts," but it hasn't done a good job of marketing these distinguishing differences.
The problem is that Google has completely failed in trying to sell Google+ as anything that's differentiated from Facebook. If you watched the movie The Social Network, you can probably recall a scene were Mark Zuckerberg said there was no room for a new social network in a world that already had Myspace. That is, until he crafted it into something unique and fresh. Google+ could take a page out of that book.