Facebook (META -0.93%) stock is firing on all cylinders lately, rising by more than 40% over the last year on the back of booming financial performance and growing optimism among investors. The company is scheduled to report earnings for the second quarter of 2015 on Wednesday, July 29, and this could be a defining moment for Facebook stock. Will Facebook keep surging after earnings or will the coming earnings report signal it's time for a pause?

On sales and earnings
Facebook is a growth business, and Wall Street is expecting sustained top-line expansion from the company. According to data compiled by Thomson Reuters, Wall Street analysts are on average forecasting $3.97 billion in total revenue during the second quarter, a year-over-year increase of 36.4%.

This would represent a sequential slowdown in growth, since total revenue during the first quarter in 2015 grew 42%, while sales in constant currency increased by a staggering 49%. It's only natural to expect decelerating growth rates as Facebook gains size over time. Besides, keeping revenue growth at over 35% per year is still spectacular performance for a business making nearly $17 billion in annual revenue. 

Mobile advertising revenue represented approximately 73% of advertising revenue for the first quarter of 2015, up from nearly 59% of advertising revenue coming from mobile in the first quarter of 2014. Mobile is a key segment in the industry, so investors may want to keep a close eye on this metric to monitor Facebook's performance.

Costs and expenses are on the rise, and this will probably hurt profit margins. Management calculates that total expenses will grow by between 55% and 65% year over year during 2015, so this is already incorporated into analysts' projections to a good degree.

Wall Street analysts estimate on average that Facebook will make $0.47 in earnings per share during the quarter, which implies a year-over-year growth rate of 12% from the second quarter in 2014. It's worth noting that earnings forecasts are quite dispersed, though --  the most optimistic estimate is for $0.59, while the lowest estimate among Wall Street analysts is for $0.39. This means that estimates go as far 26% above and 17% below the average forecast.

Beyond the headlines
Earnings and sales figures attract a lot of attention, but variables such as user growth, engagement, and monetization can be even more important. After all, Facebook needs to deliver what users and advertisers want if it's going to continue building a big and successful business.

As of the first quarter of 2015, Facebook had 1.44 billion monthly active accounts, a year-over-year increase of 13%. Daily active users stood at 936 million, growing 17% year over year during the last quarter. The fact that daily users are growing at a faster rate than monthly ones is a big positive for Facebook, as it reflects healthy engagement trends.

The company is also doing great in mobile; Facebook had 1.25 billion mobile monthly users as of the last quarter, growing 24% year over year. Daily mobile users were 798 million, expanding by 31% from the first quarter in 2014.

Not only that, Facebook is also consolidating its presence across different platforms. WhatsApp has over 700 million users, and Instagram has more than 300 million active members. Services such as Facebook Groups and Messenger are also important when it comes to cementing Facebook's relationship with users and keeping those users dynamically engaged. As of March, Facebook Groups had 700 million users and Messenger was above 600 million accounts.

In addition to a growing user base and healthy engagement from those users, investors would ideally like to see sustained improvement in monetization metrics. Average revenue per user was $2.50 during the first quarter in 2015, a big increase of 25% from $2 per user in the same period last year. Annual comparisons will be tougher in the coming earnings release, since average revenue per user stood at $2.24 in the second quarter of 2014.

Sales and earnings don't come out of thin air. Key variables such as user growth, engagement, and monetization are the ultimate drivers, and they can be crucial when it comes to determining if Facebook stock will continue to rise steeply after earnings or if it's already time for a pause.