Facebook (NASDAQ:FB) will report earnings later this week, after market close on Wednesday. The report will give investors a timely view into the social network's business. Though revenue growth has decelerated some recently, Facebook is by no means growing at a slow pace; fourth-quarter revenue was up 30% year over year.

When the company reports results, investors should check on Facebook's revenue growth rate, as well as other key metrics like average revenue per user, operating margin, and earnings per share.

Facebook CEO Mark Zuckerberg at F8 2018

Facebook CEO Mark Zuckerberg. Image source: Facebook.

Average revenue per user

Facebook's average revenue per user surged in the most recent quarter, rising 19% year over year to $7.37 -- a record high. This was driven primarily by a 30% increase in ARPU in the U.S. and a 24% increase in Europe. Facebook's Asia-Pacific and "rest of world" markets saw slower growth of 17% and 13%, respectively.

Can Facebook keep growing its ARPU in its two biggest markets (when measured by revenue) at such impressive rates? Investors shouldn't get their hopes up. Facebook CFO David Wehner said during the fourth-quarter earnings call that the social network looks poised to see ad pricing headwinds in 2019. In addition, Wehner anticipates the company's ongoing shift toward the Stories format, where Facebook's ad products are less developed, to contribute to a deceleration in growth throughout 2019.

Operating margin

As Facebook invests heavily in growth initiatives, security, and privacy, its operating margin has been narrowing significantly. Operating margin in Q4 was 46%, down from 57% in the fourth quarter of 2017.

Expect it to narrow significantly on a year-over-year basis again in Q1. On the second-quarter conference call last year, Wehner said he expects Facebook's operating margin to trend toward a percentage in the mid-thirties over the next several years. And Wehner added further clarification on the third-quarter conference call, noting that "2019 will be a big investment year. I would expect us to have the biggest change in our margin structure to happen in 2019, and for it to moderate from there."

Investors should look for an operating margin in Q1 of around 35%, down from 46% in the first quarter of 2018.

Earnings per share

Decelerating revenue and a narrowing operating margin will likely make earnings-per-share growth in Facebook's first quarter hard to come by. Indeed, flat EPS growth, or even a slight pullback in the key profitability metric, is possible.

Tune into Facebook's first-quarter earnings call after market close on Wednesday, April 24.