Less than a year after raising a $42 million Series B round, video calling service Tango has closed another $40 million round with Qualcomm as one of the series leads. Access Industries, the industrial holding company founded by Len Blavatnik, which was an investor in Tango’s last round, is also one of the leads in this round, which brings the company’s total funding to close to $100 million.

Tango may be small, but the company is gaining traction, and fast. More than 45 million people have registered for the application which is available on more than 1,000 different phones. That’s a lot if you consider it’s biggest competitor Skype is only on 65 handsets. Android is of course one of its biggest platforms, but its flagship is the iPhone, particularly the 4 and 4s — the two largest single devices in terms of both users and usage.

Tango beats Apple at its own game. It has more users than Facetime because it’s available on 3G. Facetime only works on Wifi connections. And you could say that Tango is the defacto Facetime for Android.

Speaking of Android, in Sarah’s interview with Roger McNamee, the Elevation Partners MD   had some strong words for Google regarding their mobile strategy, saying, “They passed on Skype, which had 400 million active users and could’ve been a complete mobile solution for them.”

Now, 45 million users can’t compare to Skype’s hundreds of millions. But if Facebook can buy Instagram’s 20 million users for $1 billion, perhaps Google should be taking a careful look at Tango as a possible acquisition. Not only would Google grab themselves a big chunk of the video calling landscape but, given McNamee’s view that “Android is done,” it would also strengthen the company’s presence on the iOS platform.

GGV Capital Partner Jeff Richards thinks the idea has legs. “One of the reasons that voice and video apps are starting to rise in importance is that page views are no longer what [advertisers] care about,” he explains. “They care about engagement.”

Voice and video are key tools for engagement. Google has the backend pieces, Google Hangout and video chat through Gmail and Google voice. But I don’t know anyone who’s using either of those tools. (I’ve used Hangout only once.) Tango on the other hand, I use all the time. And I know several families who use Tango rather than Skype to connect with grandparents, who want to see the little ones grow.

Altimeter Group Analyst Chris Silva says that by acquiring Tango, Google would not only inherit a large, active user base, but through using Tango as the engine behind a revamped Google+ Hangout service, “It would bolster the G+ user roles.” He adds, “The underlying idea here is that the users come for the video chat and stay for the social network’s games and tools.”

Anecdotal evidence doesn’t mean Tango is a widely used application — less than half (44%) of the service’s 45 million registered users actually use it monthly, and a mere 10% use it on a daily basis. It’s on a much smaller scale than its largest competitor, Skype. However, considering Skype has more than 600 million registered users and nearly 70% are inactive, Tango seems to be doing a much better job keeping its members engaged.  And just 18 months after launch, its daily call volume is in the single digit millions, more than doubling since December.

Mayfield Fund Managing Director Tim Chang says that although Google buying Tango would make sense, what might work against it is that the startup could simply be seen as a utility feature that Google could easily build into its next iteration of the Android Operating System. He explains, “The Facebook/Instagram deal is different, because Instagram has a truly engaged community and heavy elements of self-expression and aspirational self-identity.” Chang believes that Tango has to be able to demonstrate network effect and conversion to paid usage for a large number of users.

Tango is working on it. They’ve just announced Tango users will now be able to share video messages to everyone in their phone’s address book, whether they’re on Tango or not. They can send them to phones or emails.

The company is also building upon Tango Surprises, which launched late last year, just in time for the holidays. Tango Surprises allows users to add animations to their video conference calls with friends and families, such as fireworks, a monkey swinging from a tree, or even a kiss, with just one click. Each one costs $1.99. Over the past three months sales have grown every month and the length of calls for people who use them have doubled in length.

So today the company is announcing that it has built tools for its design partners out of China, the same ones who work with Zynga and Disney, to create animations that are optimized for 3G and can work cross-platform. They’re planning to have these designers use the tools to create enough designs to get users to look forward to new releases on a bi-weekly schedule.

These animations move Tango into the “Virtual Goods” category, which is a $2.9 billion market, According to Inside Virtual Goods. But whether or not it’s a sustainable business model for Tango is still a big question. But certainly having a patented platform to create those animations, which are instantly optimized for all platforms and 3G, is a valuable asset that Tango could even license for serious cash. Imagine if Google owned it?

Google did express interest early on in the company, according to Charles River Ventures Partner Bill Tai, who was the first investor in Tango. So, it’s not out of the question.

He says, “Tango could definitely move the needle for Google with the most sophisticated peer-to-peer technology applied to smartphones. It’s a very clever and viral user interface, and its a new monetization model. But I see it as a successful stand alone public company as well, so I don’t don’t want to see it happen.”

Altimeter Group’s Chris Silva offers a word of caution: If Google does buy Tango, don’t execute like Microsoft with Skype. He says that’s a situation “where the product continued on as a great, standalone tool with no discernible upside for Microsoft as the buyer, other than an additional revenue stream that is small as part of the greater whole.”