Google+ is seeing unparalleled growth, reaching broad audiences faster than any other digital property in history, but it remains to be seen whether Google can keep users coming back to its latest social foray.

Web analytics firm ComScore said Thursday that Google+ has already amassed  20 million unique visitors to its site, an incredible figure after being launched only three weeks ago. Of those users, five million of those are from the U.S, and the rest are international.

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 rides the back of Google CEO Larry Page's revelation that Google+ already signed up 10 million registered users. While registered users and unique visitors are not necessarily the same thing, that growth has nevertheless been impressive.

To date, there has been no strong marketing push. Growth is completely fueled by users inviting other users to join.

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.

But Google needs to take steps to ensure that this new social network doesn't follow the same fate as its predecessor, Google Buzz and Google Wave.

While those sites saw initial interest, participation waned and those projects were eventually pulled.

[Google+'s] ultimate success will depend on how well it 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.