Google's latest offering 'Google Plus' is steadily gaining ground in the social networking space, so far dominated by Facebook, with the internet giant's social network attracting about 20 million visitors in the initial three weeks and nearly half of these visitors hailing from US and India.

According to estimates by research firm ComScore, Google Plus had 19.93 million unique visitors between June 29 and July 19 period.

The top two countries in this estimate are the US and India. Of the 19.93 million visitors, 5.31 million were from the US and 2.85 million from India. Another 0.87 million users came from the UK, 0.71 million from Germany and 0.50 million from France.

"I have never seen anything grow this quickly," vice president of industry analysis at ComScore Andrew Lipsman said. Google Plus, launched on June 28, is Google's answer to rival Facebook.

It lets users post photos, messages, comments and other content from a selected groups of friends. The service was started as a field trial and the project is by invitation only, which means people can join only if a current member invites them.

Eventually, Google plans to incorporate features of Google Plus in its other services, such as its YouTube video site. Google CEO Larry Page had last week said that Google Plus had more than 10 million users.

Page said there are "more opportunities for Google today than ever before". It will be some time before Google catches up with Facebook's scale of more than 750 million users. The other popular micro-blogging site Twitter has more than 200 million registered accounts.

Through 'Google Plus', the internet giant aims to garner a share in the lucrative social networking space that has so far been dominated by the Mark Zuckerberg led popular site. The service has features like 'Circles', 'Sparks', 'Hangouts' and 'mobile'.

Through Circles, Google targets Facebook's features in which a user's information is shared by default with a large number of his or her friends including their work colleagues and acquaintances, rather than only their close personal friends.

Its "Huddle" feature lets users send text messages to several different people at once, known as group texting.