Google+ has seen dramatic growth since its launch, pulling in 20 million unique visitors within a month of its launch. But it faces several key challenges before it could become the dominant force in social media.

Online metrics company ComScore said late Thursday that of the 20 million users amassed in the three 3 weeks since its launch, five million of those are from the U.S. Google declined to comment on the figure.

Google+ is on an unprecedented growth trajectory over its first three weeks, reaching 20 million visitors faster than any site in recent memory, Comscore VP Andrew Lipsman told IBTimes.

The estimate comes a week after Google CEO Larry Page told investors that the company already had amassed 10 million registered users. While registered users and unique visitors are not necessarily the same thing, thate growth has nevertheless been impressive.

The company has not embarked on any type of marketing, aside from high-profile announcements and strategically placed statements from executives to media. Growth has been completely fueled by the invitation of existing users.

While the rate of growth is unrivaled, the social network is still small in comparison to mature rivals Facebook and Twitter, which have 750 million and 200 million registered users, respectively.

Ultimately, however, its success will depend on how Google converts this strong trial base into regular users, Lipsman warned.

Indeed, while rivals like Facebook and Twitter have become online destinations in themselves, over 50 percent of traffic coming to Google+ originates from Google or Gmail, according to Experian HitWise. Google's video site YouTube was also a large referrer.

Facebook also enjoys stronger user engagement than Google+. In the increasingly competitive digital marketing environment, advertisers are concerned with not only reach and targeting, but also user how long users are interacting with the site.

HitWise Research Director Heather Dougherty said the average visit time for Google+ last week was 5 minutes and 50 seconds, compared with almost 22 minutes users spent on Facebook.

And while Page noted that the site has already amassed 10 million users, HitWise data indicates that not nearly that many people are coming to the site, on average.

It estimates that only 1.8 million visitors from the U.S. came to the site last week -- just 10 percent of those registered.

With strong integration to Google's ecosystem of products and services and plenty of hype, Google+ has shown it has no problem attracting new registered users. The challenge will converting them from passers-by, to dedicated users.