When Google launched its social networking service, Google Plus, during the summer of 2011, tens of millions of people clamoured to sign up for an account.

But within months, critics had panned the new service, pointing to user pages bereft of meaningful content and exchanges. They said the new social site just wasn't, well, social. It seemed as though Facebook had cornered the market -- Google was too late to the party.

Perhaps not. According to data released this week by Internet analytics firm GlobalWebIndex, Google Plus is racking up large numbers of new users and continues to outpace Twitter as the world's number two social network, behind perennial titan Facebook.

The reasons behind Google Plus's growth -- it now can boast 359 million active users, up 33 percent from 269 million users at the end of June 2012, according to GlobalWebIndex -- are complex and tied to Google's effort to build a connecting layer across all its services, including search, YouTube, maps and other products. Log into one, and you've logged into the lot.

Google itself is tight-lipped about its numbers. Its last released figures were in December, when the search giant said 500 million people had created Google Plus accounts.

But of this number, only 135 million were actively posting to Google Plus pages. Millions more were using some of the service's features, such as clicking the "+1" button to show they liked certain web pages.

It remains far behind Facebook, which boasts 701 million active users, according to the report, though Facebook founder Mark Zuckerberg last year claimed more than a billion active accounts.

Still, the volume of Google Plus accounts suggests naysayers were too hasty in calling its demise. Like many social networking services, Google Plus has won over a devoted core of users.

One such convert is New Zealand photographer Trey Ratcliff, whose picture-centric Google Plus page has nearly five million followers.

"It's nice to pop into Google Plus to discover new things. Facebook is pretty good, but it's harder to discover new people or have more in-depth discussions around passions," he said in an email exchange.

Indeed, this may be how Google Plus will find its niche in the crowded social media world: Whereas Facebook is the go-to service for connecting friends, Google Plus is more often used to meet strangers who share common interests.

Google Plus acknowledged as much last year by adding its "Communities" section, which hosts a diverse mass of groups and lets users join a "hangout" -- the popular group video service.

"We're extremely happy with our progress so far, and one of our main goals is to transform the overall Google experience and make all of the services people already love faster, more relevant, and more reliable," Google said.

But some observers remain sceptical that account holders are doing much on Google Plus, and see it as little more than a tricked-out sign-in service for Google's products.

Claire Stokoe, who works as social media manager at Mediaworks, a marketing agency in the English city of Newcastle, said she is doubtful Google Plus will ever catch up to Facebook, but she warns clients not to ignore it.

"An authoritative Google Plus account is one of the factors that will help you rank high on Google (search results)," she said, noting that a popular Google Plus account was an important criterion in the search algorithm that ranks pages.

But she doesn't see the service expanding far beyond the business and marketing world -- at least for now.

"Whoever I ask, everyone has a Facebook account. I don't know anyone who has a Google Plus account unless they are in the industry, and that's because they have to," Stokoe said.

GlobalWebIndex's latest figures show that while Google Plus is the second-most popular social networking service after Facebook, Twitter is actually growing at a slightly faster clip, increasing from 206 million users at the end of June last year to about 297 million today, a rate of about 44 percent.

The study also found that usage was growing fastest among older people, especially with Twitter, confounding stereotypes that social networking is for the young.

GlobalWebIndex is a London-based firm that tracks Internet users through a series of surveys in 31 countries, with an annual sample size of about 120,000 people. It asks respondents which social platform they have directly contributed to in the last month and said Google itself uses its numbers internally.

The study found that Facebook also continued to grow rapidly, at about the same rate as Google Plus.

But the leading social network is also said to be battling "Facebook fatigue" in some countries, with some users growing bored of the service or else bemoaning its myriad changes to privacy settings and other tweaks, including the growing prevalence of sponsored content.

A study by the Pew Research Center in February found that more than half of US Facebook members had taken breaks from the leading social network. While the top reason was they were just too busy, people also cited fatigue with the service.

Judith Catterall, a retired choreographer from Portland, Oregon, said she tried to close her account after getting fed up with changes and a news feed becoming increasingly cluttered with sponsored content.

"It's one of those things where you think 'OK, I have no control,' and that may have been the final straw," she said.

But within 10 days of deactivating her account, Catterall was back on Facebook. She missed her friends.