Facebook (NASDAQ:FB) has been on a roll this year, with its stock up more than 58% in the past 12 months. People now spend more time on Facebook than any other website on the Internet, according to Morningstar. Therefore, the challenge for the social media giant going forward is how to best monetize those users. Below, three of The Motley Fool's contributors share their takes on what is most critical to Facebook's future.

Tim Brugger: Thankfully for Facebook shareholders, trying to pinpoint just one item that tops the list of the most important to continue its stellar growth is not an easy task. Why? Because there are so many different opportunities at CEO Mark Zuckerberg's finger tips. But in terms of revenue, which is what will ultimately propel Facebook's share price to new heights in the future, video is king.

As demonstrated last quarter, Facebook's emphasis on mobile is paying off big time. Of its 1.35 billion monthly average users, or MAUs, over 1.1 billion access Facebook each month from a mobile device. What does that have to do with video? Turns out, mobile users in general, and smartphone users in particular, are the most likely to purchase a product or service after viewing a video ad.

Now, combine smartphone user's affinity for video advertising with Facebook's unprecedented ability to target the right ads, to the right users, at the right time, and the revenue possibilities are endless. Facebook is closing in on unleashing PC and mobile-ready video ads across its site, and don't be surprised to find video spots popping up on Instagram before long. Facebook is already collecting a reported $1 million a day from "selected" marketing partners to simply test video ads.

As well as Facebook has done growing revenues through its targeted banner ads, video advertising provides an opportunity to take sales to a whole new level. Video, when combined with Facebook's other revenue opportunities and Zuckerberg's proven ability to execute on strategic initiatives, translates to a bright future for investors.

Alex Planes: Facebook has a lot of moving parts, but the company's success ultimately rests on its ability to boost average revenue per user, or ARPU, higher and higher with every passing quarter. I've covered ARPU in depth before, so there's no need to rehash it (click here for a detailed explanation), but we need to take a closer look at one part of Facebook's headline ARPU number: its ARPU in the rest of the world.

Facebook hasn't been an "American" website at any point in this decade -- the company began 2010 with 70% of its user base located outside of North America, and international users have since swelled to comprise 85% of all of Facebook's monthly active users. However, the revenue Facebook earns from these international users has continued to lag far behind what it earns from serving up ads to people in the U.S. and Canada:

Source: Facebook financial filings

Because of this huge reliance on revenue from its North American users, Facebook earns only 53% of its revenue from 85% of its users -- every one of Facebook's North American users is effectively worth two and a half European users, more than six Asian users, and more than eight and a half users from anywhere else in the world. If this imbalance holds up over the long term, it will certainly crimp Facebook's ability to grow, because its North American user base has more or less flattened out over the past two years. There are only 9% more North American users now than there were in 2012, compared to a two-thirds increase in Asian users and a 58% rise in users everywhere else over the same time frame.

It's probably not realistic to expect these two geographic regions to ever reach ARPU parity with North America, since the average Asian, African, and South American person has a much lower cost (and standard) of living than the typical North American. But South America, for example, has eight countries with more than 400 million total citizens that have at least about a fifth of the GDP per capita of the United States. Advertising in these countries should be able to pull Facebook's rest-of-the-world ARPU up to something closer to one-fifth of its North American ARPU, which would be $1.48. That alone would make a huge difference, raising Facebook's rest-of-the-world ARPU from $353 million to $625 million for the third quarter.

The "rise of the rest" has been a major focus in investing circles for years, but few companies stand to benefit from growing prosperity everywhere else quite like Facebook. It may not be able to boost its North American ARPU quite like it has in the past, but focusing on the rest of the world (including Asia) should be the key to Facebook's growth over the coming years.

Tamara Walsh: Many factors will contribute to Facebook's ongoing success. However, as user engagement continues to shift away from desktop toward mobile, Facebook's mobile initiatives will be of the utmost importance. As much as 83% of the company's total monthly active users connected to the social network via a mobile device during the third quarter, up from 74% a year ago. With Facebook now reaching 1.3 billion monthly active users, this means mobile will be a critical part of the social media giant's long-term success. Mobile ads didn't even exist on Facebook a year ago. Yet, revenue from mobile advertising climbed to $1.9 billion in the third quarter, thus accounting for 66% of total advertising revenue in the period. Investors will want to keep an eye on this revenue stream going forward, as mobile becomes an increasingly important part of Facebook's model.

Last month, Facebook announced a new revenue sharing program with publishers that would enable them to publish content directly on Facebook's mobile app. This would be a win for users because the content they want to read would load faster than if they were accessing it through the publisher's website. The content would feature ads that Facebook sells and the social network would then share the ad revenue with publishers, according to The New York Times.

This is a smart move because Facebook would still control the user experience, and since this third-party content would be hosted by Facebook's mobile channel most of the data collected would thereby belong to the social network. We can chalk this up to one of the many ways Facebook hopes to better monetize its mobile users going forward. Ultimately, we live in a smartphone-dominated world. Therefore, Facebook needs to continue innovating at the mobile level if it wants to keep winning.