Facebook (NASDAQ:FB) reported earnings for the fourth quarter of 2015 on Wednesday after official market hours. The bar was already quite high for Facebook leading into the report, but the social network founded by Mark Zuckerberg managed to outperform even the most demanding expectations.

Strong revenue growth
Total revenue during the fourth quarter of 2015 came in at $5.84 billion, a big 52% increase from $3.85 billion in the fourth quarter of 2014. This is quite an impressive growth rate for a company as big as Facebook, and sales also came in above Wall Street expectations, since analysts were on average forecasting $5.37 billion in revenue during the quarter.

Advertising revenue was $5.63 billion during the quarter, a 57% annual increase. Importantly, Facebook is doing really well in mobile, a crucial growth segment in the industry. Mobile advertising revenue amounted to 80% of revenue in the fourth quarter of 2015, up from 60% of revenue coming from mobile in the fourth quarter of 2014.

Facebook knows how to win friends
The company ended the quarter with 1.59 billion monthly users, a 14% annual increase. Size is a crucial variable to watch in the social-media business, because a bigger platform means more attractive content and more opportunities for interactions, so users attract each other to the big and popular social networks.

Competitor Twitter (NYSE:TWTR) is scheduled to report earnings for the fourth quarter of 2015 on Feb. 10, so user data for Twitter as of December is still not yet available. However, Twitter reported only 308 million average monthly users in the third quarter of 2015, and Twitter's user base grew by a modest 8% year over year during the period. In this context, Facebook's gargantuan scale looks even more impressive when comparing the company against other industry players such as Twitter.

Not only that, but Facebook is also growing where it really counts. Monthly mobile users were 1.44 billion as of the end of 2015, a 21% annual increase. Engagement is also trending in the right direction, as the company reported 1.04 billion daily users, a 17% increase. Mobile daily users stood at 934 million for December, growing by a strong 25% year over year.

Management is also doing a great job at driving increasing monetization levels. Facebook made $3.73 per user on a worldwide scale during the fourth quarter, a 32% increase from $2.81 per user in the fourth quarter of 2015. Revenue per user in the U.S. and Canada is significantly higher than in the rest of the world, at $13.54. This metric shows that Facebook still has a lot of room to continue increasing monetization on a global scale if it's going to bring monetization in different markets to levels closer to those in the U.S. and Canada.

Expanding margins and growing earnings
Facebook is aggressively investing for growth. Total costs and expenses grew 21% year over year, reaching $3.28 billion. However, revenues are still outgrowing costs, so operating margin expanded from 29% of sales in the fourth quarter of 2014 to a big 44% of revenue in the fourth quarter of 2015.

Because of explosive revenue growth and increasing profit margins, overall net income jumped by 123%, reaching $1.56 billion. Earnings per share amounted to $0.54 during the quarter, more than doubling from $0.25 per share in the fourth quarter of 2014. 

Investors had an aggressively optimistic reaction to the news, as shares of Facebook were up by more than 12% on Wednesday after official market hours. Considering that sales, earnings, users, and engagement are all moving in the right direction, it's no wonder Facebook's status update is getting so many likes from investors.

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