Facebook Co-Founder and CEO Mark Zuckerberg has said ad nauseum that the social network is becoming a mobile company. It appears that the statement has finally come true. As announced during Wednesday’s fourth-quarter earnings report, Facebook now has more people accessing the site via phones and tablets than via desktop. Zuckerberg said (but did not elaborate) during the earnings call that the company is developing mobile-first projects that will make the experience better for both users and advertisers.

Of Facebook’s 1 billion-plus monthly average users, 618 million check Facebook from their phones or tablets. Even more interestingly, Facebook announced that 157 million MAUs only access Facebook from their mobile devices, and that number has grown quite a bit over the past year. In the fourth quarter of 2011, just 58 million MAUs were mobile-only:

People aren’t just checking their Facebook updates on their mobile devices — they’re engaging rapidly.

According to comScore statistics in December, Facebook is the top driver of awareness of new mobile application installs. Among those who learn about new apps on Facebook, 48 percent click directly from the Facebook app to download new mobile apps. Zuckerberg said that between the Facebook mobile app and Instagram, Facebook now accounts for more than one-quarter of all mobile activity.

Facebook has made major strides in becoming a company that thrives on mobile, but there’s still plenty of work to be done. At various times throughout Wednesday’s call, Zuckerberg hinted that Facebook is working on mobile-first experiences (although this does definitely not mean a phone), but when various investors tried to get him to say more, he remained mum:

I think more people are starting to understand that mobile is a great opportunity for us. Mobile is the perfect device for Facebook for three reasons — it allows us to reach more people, get more engagement from the people who we reach, and I think we’ll also be able to make more money for each minute that people spend with us on their mobile devices … The next thing we’re going to do is get really good at building mobile-first experiences. That’s going to be a big thing for us this year. If we do this well, we should be able to bring even more relevant content and connections to more people and continue to deepen their engagement.

Of course, while mobile is a great platform for bringing Facebook users together from all over the world, it also has to be a viable way to advertise. Facebook proved that late in the fiscal year. The company’s mobile advertising revenue rose from 14 percent in the third quarter to 23 percent, nearly one-quarter of the overall ad revenue intake, in the fourth quarter.

Facebook Chief Operating Officer Sheryl Sandberg pointed to a successful Facebook mobile campaign that Walmart ran over the holidays, which delivered 50 million ads to potential customers. Sandberg also noted that more brands are using Facebook to launch new products, generating buzz that leads to sales. Michael Kors utilized the social network to launch a new line of sneakers. The company said that many of the shoes sold out online and in stores, and Michael Kors achieved a 16-point increase in awareness in the new sneaker among the 36 million people who saw the campaign on Facebook. That’s basically 5.8 million new people in the brand’s target audience who are now aware of those kicks.

Sandberg noted that Facebook has something truly powerful — millions upon millions of people using their true identities on Facebook — so brands have the chance to serve ads that are relevant:

We’re continuing to take advantage of the significant opportunity we have on mobile. As we said before, approximately 23 percent of our advertising revenue now comes from mobile. In addition, 65 percent of our advertisers are now using ads in news feed, which run on both desktop and mobile, up from 50 percent at the end of the third quarter. Marketers are recognizing that our news feed is the most efficient and effective place to reach their customers … Clients also recognize that because our users share their real identities on Facebook, and because they are logged in when they use Facebook on mobile, we have a unique ability to serve advertising that people find relevant. This is an important competitive advantage for us, relative to other mobile platforms.

Readers: What mobile-first opportunities do you think Facebook is working on?