Facebook (NASDAQ:FB) stock has been a high performer recently. Shares surged 57% in 2019, crushing the S&P 500's 29% gain over the same timeframe. This bullish run, of course, means the social network has a high bar to live up to -- a bar it didn't quite meet on Wednesday.

While the company's fourth-quarter results were notable, they apparently weren't impressive enough to justify the tech stock's current valuation. Shares sold off more than 7% in after-hours trading following the company's fourth-quarter update.

But investors should think twice before they cash out. The company continues to demonstrate robust revenue and user growth while raking in substantial free cash flow.

Here's a closer look at the results.

Facebook CEO Mark Zuckerberg discusses the company's 10-year roadmap at F8 2018

Facebook CEO Mark Zuckerberg. Image source: Facebook.

Facebook's fourth-quarter financials

Facebook reported revenue of $21.1 billion, up 25% year over year. This handily beat analysts' average forecast for revenue of $20.9 billion. It was also slightly ahead of management's guidance for revenue growth between 20% and 24% year over year. The market, however, may have been hoping for more rapid revenue growth given how the company's top line stubbornly resisted deceleration in the two prior quarters, despite management guiding for revenue growth deceleration throughout 2019. In other words, some investors may have thought Facebook was lowballing its guidance for Q4.

Earnings per share also beat estimates, coming in at $2.56 -- $0.03 ahead of analysts' average forecast. But earnings per share were only up 8% year over year thanks to the company's outsized growth in expenses. Facebook's expenses increased 34% year over year to $12.2 billion, resulting in a narrower operating margin of 42% compared to 46% in the year-ago quarter.

Free cash flow during the period was $4.8 billion -- up from $3.3 billion in the year-ago period. For the full year of 2019, free cash flow was $20.7 billion, up from $15.4 billion in 2018.

Healthy user metrics

The social network also revealed more steady growth in its user metrics.

Monthly active users on its core Facebook platform were 2.50 billion in Q4, up 8% year over year and higher than the 2.45 billion it had in the third quarter of 2019. Daily active users increased 9% year over year to 1.66 billion.

Facebook also saw more strong growth in its family metrics, a measure of active users across all of its services that attempts to de-duplicate people so that they are not counted more than once on its platforms. The company refers to this metric as family monthly active people (MAP). MAP rose 9% year over year to 2.89 billion. Notably, this was also up from 2.82 billion in the third quarter of 2019. Family daily active people (DAP) increased 11% year over year to 2.26 billion.

The social network also revealed a new metric: family average revenue per person, or ARPP. Previously, Facebook only broke out average revenue per user on its core Facebook app. This key metric came out to $7.38, up 13% year over year.

With Facebook's users growing nicely and ARPP rising even faster than users, the social network looks poised for more strong growth in 2020.