People around the world watched more than twice the amount of streaming video in 2017 compared to 2016. It's no surprise the number of streaming hours grew last year, but what's interesting is that the growth in watch time accelerated year over year, according to a study from Conviva.

That's great news for companies already deeply entrenched in streaming video like Netflix (NASDAQ:NFLX), Amazon (NASDAQ:AMZN), and Roku (NASDAQ:ROKU). It's also a sign of the challenges facing traditional pay-TV providers like AT&T (NYSE:T) and Comcast (NASDAQ:CMCSA).

As consumers get even more options for content, Conviva expects the growth in streaming hours to keep accelerating this year. Here's what that means for video content providers and investors.

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Streaming is still concentrated in North America

Most streaming video hours took place in North America. Fifty-eight percent of total watch time on streaming services came from the United States, Canada, and Mexico -- just 21% came from Europe and another 19% from Asia.

The United States is thought to be reaching saturation in terms of streaming subscribers. Still, Netflix added 4.9 million new subscribers in the U.S. over the past four quarters -- up about 10%. Amazon doesn't release subscriber counts, but analysts believe Prime might not have very many U.S. households left to capture.

Nonetheless, the hours spent streaming continues to climb. Prime Video set a new single-day streaming record on Dec. 30 last year, and Amazon says Prime members used their digital benefits more than any other year in 2017.

There's actually plenty of room for an increase in streaming hours. The average adult still spends nearly 4.5 hours daily watching live TV, according to Nielsen data. So while total subscription growth might be slowing in the U.S., consumption time is still shifting toward streaming as the content available to stream keeps getting better.

Traditional pay-TV is getting into streaming as well

The concentration of streaming hours in North America may also be the result of more traditional pay-TV providers tapping into the market. Comcast markets its on-demand streaming content that subscribers can access through Xfinity.com, and most other cable and satellite companies offer similar services.

Last year also saw wider acceptance of over-the-top cable replacements. AT&T launched DIRECTV Now at the end of 2016, and it had over 1 million subscribers by the end of 2017 (not to mention Sling TV and PlayStation Vue).

Virtual pay-TV services like DIRECTV Now allow traditional providers to reach a new audience and fix certain pain points for potential subscribers. There's no waiting for an installation, no long-term contracts, and the pricing is generally transparent. That also cuts out costs for the providers, allowing them to make better offers compared to their legacy services.

But the rise of virtual cable also opens the door for new competition, and it's fierce. Without the need to invest in tons of physical infrastructure, dozens of tech companies have been able to compete with old school pay-TV companies after negotiating contracts with content creators.

So where's the opportunity for investors?

As streaming video hours continue to accelerate, and streaming options expand, there's a more ways than ever for investors to get in on the action.

While Netflix and Amazon are starting to see slower subscriber growth in the U.S., both companies still have lots of opportunities for international growth. But streaming services that leverage advertisements may be a bigger opportunity. Roku is one of the best options to capitalize on the streaming trend. Any new service -- whether it's a start-up or a major cable company -- will benefit the overall platform. More streaming took place on devices like the Roku than on computers and mobile devices combined, according to Conviva.

And international markets are also attractive considering just 21% of streaming hours came from Asia -- a market that accounts for about two-thirds of the world's population. Baidu is looking to take iQiyi, its video-streaming platform, public this year. HotStar in India has millions of viewers, but it's owned by Twenty-First Century Fox so not exactly a pure play in streaming video.

Despite saturation in established markets, the acceleration of growth in time spent streaming around the world could produce excellent returns for the many players in this space.

John Mackey, CEO of Whole Foods Market, an Amazon subsidiary, is a member of The Motley Fool's board of directors. Adam Levy owns shares of Amazon. The Motley Fool owns shares of and recommends Amazon, Baidu, and Netflix. The Motley Fool recommends Comcast. The Motley Fool has a disclosure policy.