The future of Google’s greatest social networking effort to date, Google+, may be debatable, but the search giant hasn’t found it very difficult to lure brands to Google+.

And for good reason: Google+ has been Google’s most respectable social effort to date and brands have learned that getting on board services before they get big is often a far better strategy than waiting until it’s too late.

While it remains to be seen whether those brands that joined the Google+ bandwagon early will be rewarded with ROI, there is some promising news according to social media analytics provider Simply Measured.

It looked at the Interbrand Top 100 brands that are present on Google+ and found that, six months in, 22% of the brands circler counts exceeding 100,000. Nike, which joined just two months ago, is one of them. All told, the total number of circlers counted has grown 138% since Simply Measured’s first report three months ago, and circler engagement is up 112%. Engagement with content, which Simply Measured says is driven primarily by photos and videos, is up too, although not as much (65%).

As one might expect, not all brands and verticals are treated equally on Google+. Four luxury brands — Ferrari, Gucci, H&M and Burberry — are tops in terms of the number of circlers they have. Combined, they have more than 2.5m circlers. In terms of verticals, brands in the automotive, electronics and luxury categories are by far the most popular.

According to Simply Measured CEO Adam Schoenfeld, “With a user base surpassing 100 million and growing fast, Google+ is becoming an attractive channel for brands to engage with consumers.”

For brands like Ferrari and Gucci, that does appear to be the case. But there’s a wide chasm outside of the top-tiers of the Google+ brand page leaders. Armani, which ranks twenty-second in circlers with 111,000, is followed by Adidas, which has just 26,000 followers.

A lot of this has to with how great an effort different brands are making on Google+, which raises the question: just how much time and money should brands invest in Google’s social network to get results? Ferarri’s 730,000 circler count is impressive, but the iconic automobile manufacturer has more than 8.1m likes on Facebook. Adidas, which has a far less impressive circler count, has an equally impressive 7.6m likes on Facebook.

Obviously, Google+ isn’t yet a year old, so comparing circlers to likes isn’t entirely fair. But at the end of the day, savvy brands will begin to make comparisons. What’s the cost of acquiring a new circler compared to a like? At what point does fan acquisition hit a plateau on both networks? What’s the value of circler versus a Facebook fan?

Right now, Google can’t offer a Facebook-sized audience, so it should probably focus on doing what it can to ensure the answers to these questions cast Google+ in a favorable light.