91Èȱ¬

Google+ is being opened up for anyone to join after two-and-a-half months in closed testing.

The search firm's latest foray into social networking was initially offered to journalists and people working in technology related fields.

However, members' ability to invite friends meant its user base quickly grew to tens of millions.

Google+ has been praised for several innovative features including its multi person "hangouts" video chat.

Two weeks after it launched, the company announced that its service had attracted 10 million users.

It has not produced official figures since then, although estimates by web analytic firm Comscore put membership at 25 million by the end of the first month.

Despite users' apparently rapid uptake, by Australian web software maker 89n suggested that the rate of public posting was declining. The survey did not take account of private postings.

Alongside its full public launch, Google is adding several new features to the service, including video hangouts on Android smartphones - allowing multi-person chats.

Users will also be able to share the content of their computer screens with whoever they are talking to.

Another notable addition is the integration of search within Google+, allowing members to find results from socially shared information, as well as across the wider web.

The prospect of "social search" through Google would appeal to businesses, according to social networking specialist Matt Rhodes from Fresh Networks.

"Brands know that a lot of people who come to them come through search and anything they can do make themselves come up through the right terms or higher up the rankings is important. Commercially that is the opportunity," he said.

"If you are logged into Google and search for supermarkets, if some of my friends have plus 1'd Tesco or commented about it, that might push Tesco above Sainsbury's in my search results."

Myspace founder Tom Anderson - an active Google+ user - told 91Èȱ¬ News: "Failure is not an option for Google on this. The fate of their business depends on social in the long run, so I think they'll keep tweaking and reminding users until they get it right."

The rise of Google+ has not gone un-noticed by its competitors.

Facebook has recently rolled out a number of innovations which many observers have characterised as a direct response to the Google challenge, although the company argues that these have been in the pipeline for far longer.

These include a revamp of the social network's friend system, making in easier to add people to categories, similar to Google Circles.

Earlier this week it was revealed that Facebook users would be able to connect their status updates directly to their Twitter feed, in a move that may enhance the appeal of both Google rivals.

Conversely, Google finds itself in the unusual position of being a minority player in the market, facing Twitter's 200 million users and Facebook's 750 million.

Vic Gundotra, Google's senior vice president of social business told the 91Èȱ¬ that he welcomed the competition.

"We suspect people use many different tools to share," he said.

"The most popular mechanism people use to share today is email. So we think there's lots of room for innovation."