Facebook (NASDAQ:FB) has been one of the biggest winners in the market over the past year. Even after a substantial pullback due to general market weakness in the last couple of weeks, Facebook stock is still up by over 30% from its lows of the last 12 months. The company is scheduled to report earnings for the fourth quarter of 2015 on Wednesday, Jan. 27, and investors may want to keep a close eye on these three crucial areas.

It's all about users
Facebook has reached gargantuan scale, with 1.55 billion average monthly active users as of September, a healthy increase of 14% year over year. This is a crucial advantage in the industry, since a bigger social network provides more opportunities for valuable interactions and users attract others to the platform.

To put the numbers in perspective, Facebook is considerably bigger than competitors such as Twitter (NYSE:TWTR) and LinkedIn (NYSE:LNKD.DL). Twitter has been facing considerable difficulties in user growth, and management is aggressively working on multiple initiatives to jump-start performance. Twitter ended the third quarter with 308 million average monthly users when excluding SMS Fast Followers, and the user base grew only 8% versus the third quarter in 2014.

LinkedIn has a different business model, as opposed to online advertising. Most of its sales and earnings come from the fees it charges to companies and premium users in exchange for access to its human resources solutions. Since LinkedIn is focused on a specific niche, it should have a smaller potential market than does Facebook or Twitter. Nevertheless, LinkedIn is still doing well on the user front, reporting 396 million members in the third quarter, an annual increase of 20%.

The main point is that Facebook is the biggest player in the social media industry by a wide margin, and this is a key source of competitive strength for the company. While it makes sense to expect growth to slow down as the platform gains size over time, watching Facebook's ability to continue attracting more users is of utmost importance.

It's not just about the user count. Facebook also needs to keep those users actively engaged in the platform in order to build a sustainable and growing business. According to several reports, millennials and young users are increasingly spending their time in trendier social networks such as Snapchat. This is only natural. After all, Facebook can't be the most popular social network for both parents and teenagers at the same time. However, investors would ideally like to see rising overall engagement levels.

Facebook has 1.01 billion daily active users as of September, which represents a strong year-over-year increase of 17%. Since daily users are growing at a faster rate than monthly ones, this shows that user engagement is moving in the right direction. 

The company is also gaining ground in the very important mobile segment. Facebook has 1.39 billion mobile monthly users as of September, a 23% annual increase. Mobile daily users jumped by a vigorous 27% year over year last quarter, reaching 894 million.

Facebook is also expanding into an ecosystem of services and applications. WhatsApp is approaching 1 billion users, and management said in the third-quarter conference call that Messenger and Instagram have over 700 million and 400 million users, respectively. This is a major positive for investors, as it means additional opportunities for growth in the years ahead.

Monetization is a key variable in terms of overall financial performance. The company made $2.97 in revenue per user last quarter, a big increase of 24% versus $2.40 per user in the third quarter of 2014. 

Importantly, Facebook is making more money per user while reducing the overall amount of ads in the platform, and this is a big plus in terms of improving monetization while taking care of the user experience at the same time. Based on data for the third quarter, total ad impressions declined by 62% year over year, while the effective price per ad jumped by a vigorous 61%. 

Better targeted ads allow the company to create more value for advertisers without alienating users with too much advertising, so this is a key trend, not only for users and advertisers but also for investors in Facebook stock.