Based on its steady, increasing stock price leading up to Q3 earnings on Nov. 4, not to mention its ongoing march to yet another 52-week high following the news, it's clear Facebook (NASDAQ:FB) investors got what they were expecting last quarter: another home run. About the only "negative" was Facebook's continually declining desktop-gaming revenue, which was largely responsible for the 18% drop in its payments and other fees unit to $202 million from last year's $246 million.

Of course, "other fees" make up such a small portion of Facebook's $4.5 billion in total sales -- a 41% jump compared to the year-ago period -- that it hardly matters. Amongst all the good tidings Facebook shared in Q3, there were a few areas in particular that really stood out that investors would be wise to keep an eye on going forward.

Record levels of engagement
Despite its size relative to Twitter (NYSE:TWTR), Facebook's continued user growth is staggering. With 1.55 billion monthly average users (MAUs) as of last quarter, Facebook actually grew its total user base 14%, nearly twice that of struggling Twitter. An amazing feat and one that should continue given CEO Mark Zuckerberg's initiative is already in 29 countries and is responsible for bringing "more than 15 million people online."

As impressive as Facebook's total number of users is, it pales in comparison to the over 1 billion people that access Facebook every day. Though Twitter doesn't release daily average user (DAU) results, there's no better way to measure user engagement and Facebook's are off the charts.

Zuckerberg's willingness to tinker with the user experience is clearly working and engaged users are an advertisers dream. Just recently, Zuckerberg and team have improved Facebook's mobile profile design --  including videos and not just photos -- introduced a new version of its "like" button for more expressive responses, incorporated Groups, and added an enhanced search function to name but a few developments. MAUs are nice, but it's DAUs that really matter.

Users on the go
Just a couple of years ago, industry pundits were bemoaning Facebook's lack of a mobile strategy. As the proliferation of mobile devices increased, the concerns were that Facebook would lose out to folks like Twitter, which already boasted a strong mobile presence. In fact, about 80% of Twitter's small-ish MAU base are of the mobile variety.

Turns out, the pundits needn't have worried about Facebook. As of last quarter, just shy of 1.4 billion of Facebook's 1.55 billion MAUs accessed the site on the go. Akin to the aforementioned user engagement levels, impressively almost 900 million DAUs accessed Facebook via a mobile device in September, a whopping 27% improvement over last year.

More than a billion Android users access Facebook monthly via their device, helping to drive a 61% increase in average ad prices in Q3. The proliferation of mobile also played a role in increasing the number of advertisers using Facebook or one of its properties including Instagram. Facebook now boasts "over 2.5 million paid advertisers," which is up from Q2's "more than 2 million." Is that all thanks to mobile? No, but it's certainly playing a significant role as evidenced by Facebook generating nearly 80% of its total ad revenue via its mobile users, up from 66% a year ago.

Spend money to make money
To no one's surprise, overhead jumped again last quarter with total costs and expenses climbing 68% year over year to $3 billion from last year's $1.8 billion. And as CFO Dave Wehner made clear again in Q3, more spending is on the horizon.

Wehner said capital expenditures, stock-based compensation, and Facebook's fast-growing workforce -- there are now nearly 12,000 employees, up 44% over last year -- will continue to drive increased spending. As Wehner noted in earlier conference calls, investors can expect 2015's annual expenses to increase about 55% compared to last year.

Assuming Facebook continues its impressive results in these key areas, investors can expect more quarterly home runs going forward. And with Instagram hitting its stride, WhatsApp nearing 1 billion MAUs, Messenger and its more than 700 million users, and the Oculus Rift virtual reality headset nearly ready for prime time, the question now is when -- not if -- will Facebook hit $150 a share?

Tim Brugger has no position in any stocks mentioned. The Motley Fool owns shares of and recommends Facebook and Twitter. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.