Ad targeting. Google+ is designed to power ad targeting, and for that it only needs you to sign up once. This lets it combine the biographical information you initially enter such as age, gender, education, employers, and places you’ve lived with your activity on Search, Gmail, Maps and all its other products to create an accurate identity profile. And this powers targeting of more relevant ads it can charge more for.

So despite comScore showing that the average Google+ user only spends 3 minutes per month on Google+, VP Bradley Horowitz wasn’t lying when he told the Wall Street Journal “We’re growing by every metric we care about”.

Maybe when it first launched, Google+ had aspirations of stealing away some of your content feed reading time from Facebook and Twitter. While it needs a lot of work, the design and features Google+ have launched are solid, and I have the utmost respect for a team doing the best it can. The problem is that it doesn’t solve a problem. Facebook owns the social graph and the relevance-sorted news feed of your friends’ activity, and Twitter owns the interest graph and the firehose of news and real-time updates.

But that was not why Google made building social functionality a priority. Nor was improving its already dominant search feature. It’d would love this engagement but it doesn’t need it. Google scrambled to build Google+ because it watched Facebook and saw users were willing to volunteer biographical data to their social network, and that data is crucial to serving accurate ads users want to click. Search keywords and algorithmic analysis of your Gmail and other content weren’t enough. It had to start the journey to identity after shortsighted years of allowing users to sign up without asking who they really were. 90 million signups is a good start.

Isn’t it curious that Google+ doesn’t actually show you any ads? It’s because the time-on-site and page views there are trivial. Hit the road, Jack. Don’t you ever come back and post an update, upload a photo, or add anyone to your Circles. It doesn’t matter. What’s important to Google is getting your biographical data. That’s why Google founder Larry Page said that by “baking identity into all of our products…you’ll have better, more relevant search results and ads.” on the 2011 Q3 earnings call.

But the really sad part is that soon, biographical data won’t be enough either. Facebook, having successfully determined who over 845 million people are, now wants to know what they do. It wants behavior data, and has created the Open Graph platform to foster apps that tell it what you do in structured way it can target ads against.

Google’s a talented company with a trove of Android usage, search, email, and maps data Facebook and Twitter lack. But now it needs to think hard, come up with a brilliant strategy, and leap-frog. Otherwise Google is going to find itself playing catch up in a race that’s already over.

[Image Credit: Remixed from Emily The Strange]