It is a question CEO Mark Zuckerberg must be tired of hearing: "When will Facebook (NASDAQ:FB) begin to monetize Instagram?" It seemed every conference call following yet another quarter of stellar earnings included the aforementioned question. And why not? The fast-growing site seems perfect for advertising, leaving investors and industry pundits wondering when Facebook would "unleash" Instagram's revenue potential.

Admittedly, it has been about three and a half years since Facebook dropped $1 billion for the then up-and-coming app, so getting a return on that investment was long overdue in many investor's minds. Though Zuckerberg and Facebook COO Sheryl Sandberg urged patience after finally taking the advertising wraps off Instagram this past summer, that hasn't prevented industry insiders from forecasting its impact on revenue, and those estimates are through the roof.

Turns out, based on some new data, even the sky-high expectations of what Instagram will contribute to the top line may prove conservative.

How big is big?
As Facebook nears its third quarter earnings call slated for Nov. 4, it's not likely that Instagram will make much of a dent. As Sandberg alluded to, it will take some time before the top line enjoys an appreciable, Instagram-driven sales hike. That said, analyst estimates for this year suggest Instagram could add as much as $700 million in revenue to the fiscal year.

The predictions for Instagram don't stop with 2015. By 2020, Instagram will generate nearly $6 billion in annual revenue, thanks in part to its monthly average users (MAUs) climbing to 680 million from its current 400 million plus.

To put Instagram's projected sales growth into perspective, according to estimates, Alphabet's wildly popular YouTube property will kick in about $1.55 billion in net revenue this year, climbing to $2 billion by 2017. Considering YouTube enjoys an audience of over one billion users, and its relatively long history compared to Instagram, expectations for Facebook's photo-sharing property may seem overly optimistic but then again, maybe not.

All aboard
A survey was recently conducted asking marketing professionals across the U.S. which of the "new" visual social media sites -- including Pinterest, Snapchat and Tumblr, among others -- they were most likely to allocate ad dollars to. The results were overwhelmingly in Instagram's favor. In fact, 72% of the folks responsible for ad spending said they were "interested in allocating money to Instagram for advertising." Next on the list was Pinterest, with 41% saying they will consider it for ads.

Advertiser's love of Instagram is why research company eMarketer expects Facebook's property to generate about $1.5 billion in sales next year, more than doubling this year's take. Alphabet's YouTube, by comparison, is slated for an estimated $1.88 billion in net revenue in 2016. As evidenced by the survey of U.S. marketers and their affinity for Instagram, it could overtake YouTube revenue in just a couple of years.

What's the big deal?
When Facebook first began toying with video ads over a year ago, it reportedly charged its advertising partners lucky enough to participate in the trials a cool $1 million a day for the privilege. Though Sandberg was reluctant to disclose just how much revenue video spots were adding to Facebook's coffers last quarter, she did note they remain "a priority", and with the rates video ads command, it's no wonder.

Video is at least part of the reason Instagram revenue estimates in the coming years are so high. Being a photo-sharing app, video spots are logical. Not to mention, a significant number of the over 400 million Instagram MAUs are of the mobile variety, which suits Facebook's own mobile ambitions perfectly. Last quarter, for example, Facebook CFO Dave Wehner said that mobile ad rates jumped 220%, and that was before Instagram spots in general, and video ads in particular, went mainstream.

As Facebook stock inches ever closer to the vaunted $100 a share level, some investors may think it's a bit late to enjoy the ride. Don't. Along with Facebook's laundry list of other future growth drivers, including its Oculus Rift virtual reality headset and WhatsApp, Instagram will continue to push sales higher -- perhaps even more than the already-impressive estimates suggest.