A report out this week from ComScore tells us that Google+ users are unengaged.
According to the report, users are signing up, then leaving. Google+ users are on the service just a tiny fraction of the time that Facebook users are. For internet marketers, however, the ComScore numbers don't tell the story. Google+ deserves your attention for reasons completely different than mass reach (and in addition to the benefits it ads to your SEO strategy).

Since its launch in June of 2011, over 90 million users have registered on Google+, approximately 10% of the number of people registered on Facebook. And, according to the report, the average Google+ user is on the service on 3 minutes each month compared with 7 hours per month for Facebook users. Marketers that have been allowed to test the Google+ service are reporting that users are far more engaged with them on Facebook. For these reasons, the Wall Street Journal reports that Google+ is virtual ghost town.
Whether Google+ was designed to be Facebook killer is for someone else to decide, but my experience on the two services (and Twitter and LinkedIn) tell me that looking at average user statistics on Google+ will leave you misinformed. Google+ is clearly not yet accepted by the "average" social media user. And that is perfectly fine. Marketers should use Facebook to reach the "average" user.
For marketers who look beyond "averages," Google+ serves a purpose that Facebook can't. Sure, use Facebook to offer my Aunt Edna a coupon or a new recipe, but use Google+ to engage in thoughtful discussion. Use Facebook to generate pithy comments and run promotions, but use Google+ to have a grown-up conversation about the merits of your marketing strategy.
If you're reading this on my Google+ page, you'll see below a Google+ circle of 500 users, put together by Robert Scoble, that you'll be hard pressed to interact with on Facebook. These are business and thought leaders who actively share and interact on Google+. Read their posts, comment, add them to your circles, and I promise that the knowledge you gain will be well worth your time.
Edit 2-29-12: Here's another perspective from TechCrunch worth reading.