If Google+ looks like a ghost town to you, Vic Gundotra, Google’s svp of engineering, thinks the problem isn’t his social service, but you.

During a one-on-one conversation at the South by Southwest Interactive conference, blogger Guy Kawasaki asked Gundotra about recent criticism that Google+ isn’t getting much traction.

His response: “Make sure you’re using it correctly.” (Sounds a bit like Steve Jobs during antennagate, no?)

The majority of content is shared privately to circles, Gundotra said, adding that if you visit a page and it looks empty, “maybe they’re just not into you yet.”

The industry may have the impression that Google+ is languishing, but Gundotra said it's the fastest growing service Google has ever released and now gets 100 million active users over a 30-day period.

Gundotra also took a subtle dig at Facebook’s approach to advertising.

“We think we should not be injecting ads into your most intimate social experiences,” he said.

Instead of surfacing ads while people are looking at family photos or pics from a best friend’s party, he continued, Google’s approach “marries the best of social with the best of commercial intent.”

That means ads won’t appear on Google+, but when people are searching and, through Google’s social search features, see that a friend has +1’d a business or restaurant.

And, he added, early Google data now shows that socially annotated ads (those that have been +1’d) get a 5 percent to 10 percent uplift in clickthrough rates.

“That allows us to not jam ads into photos of your daughter,” he said. “It’s serving the ads when they’re relevant.”