YouTube viewership hit a milestone last year, the company said Monday, with global users now collectively watching a billion hours of video a day.

As the Wall Street Journal first reported, the site reached that milestone in part by powering its recommendations engine with machine learning algorithms. Other factors have contributed to the increase in viewership, including the shift to mobile: A growing number of people have internet access via their mobile devices, and more than 60 percent of YouTube watch time now happens on mobile and tablets.

In 2012, YouTube changed the way it measured viewership, switching from the number of clicks a video receives to the time spent watching videos. Since then, viewership is up 10-fold. As YouTube said on its blog, "we thought the amount of time someone spent watching a video was a better way to understand whether a viewer really enjoyed it."

The Alphabet-owned company appears to have a leg up on other platforms: Facebook reported just over a year ago that its users watch 100 million hours of video a day, while Netflix said its users watched more than 116 million hours a day in 2015. Meanwhile, as the Journal pointed out, YouTube's global popularity almost matches the popularity of television in the US: Nielsen data indicates Americans watch around 1.25 billion hours of live and recorded TV a day.

Alphabet doesn't disclose in its earnings reports how much revenue the video-sharing site brings in. However, facing new competition like Amazon Video Direct, the company last year made the case that the platform provides a robust revenue stream for content creators.

Meanwhile, as part of its efforts to boost global viewership, the company last fall introduced a lightweight YouTube app for India.