What on earth could send shares of a seemingly overvalued company skyrocketing 15% in after-hours trading? Accelerating growth rates and mobile domination.

Posting big numbers where it matters most
Talk about a blowout quarter. Surprises don't get much better than Facebook's (NASDAQ:FB) second-quarter results today. Facebook was expected to deliver EPS of $0.14 and revenue of $1.62 billion. Instead, the company reported EPS of $0.19 on revenue of $1.81 billion. Let's take a close look at the important metrics.

Accelerating growth rates. Last quarter, Facebook's overall revenue was up 38% from the year-ago quarter. Its advertising segment, which contributes the majority of the company's revenue, delivered a nice year-over-year gain of 43%. Going into the second-quarter results, I was hoping Facebook would at least maintain this handsome growth rate.

Turns out, I was much too conservative. Overall revenue growth was up 53% from the year-ago quarter. Advertising revenue (88% of total revenue) soared 61%. Facebook's revenue growth rate meaningfully accelerated.

How did Facebook pull this off?

Growth in mobile. Not only did Facebook continue to boost its active mobile users, it is also effectively monetizing them.

Last quarter, Facebook's mobile monthly active users, or mobile MAUs, accounted for 68% of Facebook's total MAUs, yet mobile advertising revenue accounted for just 30% of total advertising revenue. Though Facebook's sudden success at monetizing mobile was impressive, it wasn't certain to what extent mobile MAUs could be monetized.

One quarter later, investors have a vastly larger foundation to build their projections on. Sequentially, mobile MAUs as a percentage of total MAUs were up slightly, at 71%. But mobile advertising revenue was up considerably, accounting for 41% of total revenue. Just three quarters ago, mobile advertising accounted for just 14% of total advertising revenue.

The once unanswered question of whether or not Facebook can monetize mobile now has a clear and stern response: Absolutely.

Other key metrics
Though Facebook's success in mobile was definitely the highlight of the day, I had my eyes on several other key metrics when the company reported earnings. In particular, average revenue per user and engagement rates.

A 53% boost to revenue is definitely impressive. But was it simply a result of the network effect's compelling invitation to new members? Or was it more effective monetization?

Though new members did join the addictive network, Facebook's revenue boost was mostly due to better monetization. Average revenue per user in the U.S. and Canada was up 27%, year over year. Worldwide, average revenue per user increased 24% during the same period. Last quarter, Facebook's growth rates in average revenue per user for the U.S. and Canada and worldwide was just 20.6% and 11.5%, respectively.

In short, Facebook's redesigned timeline and newsfeed and its evolving ad business seems to be working.

But is all this revenue coming at the expense of the user experience? Nope.

Facebook's engagement rate, or active users as a percentage of monthly active users continued to climb. Up to 71.2%, more Facebook users are using Facebook every day than ever before. In the year-ago quarter Facebook's engagement rate was substantially lower, at 58%.

Was there any bad news?
Mostly, Facebook offered impressive news on all fronts. But one area did show signs of a slowdown. Facebook's growth in total MAUs decelerated to 21% year-over-year growth, down from 23% last quarter. If MAU growth continues to decelerate, year-over-year revenue growth comps could get tougher in the future. Still, 21% is nothing to sweat about.

There's really no way around it: Facebook is still in its prime.

Time to buy?
Unfortunately, Facebook isn't an ounce of a better deal today than it was yesterday. In a matter of minutes, bullish investors boosted Facebook's market capitalization by a whopping $10 billion. Though the results were downright awesome, the stock's respective gain has already taken on the quarter's glory.

So, cheers to Facebook shareholders -- your business is firing on all cylinders. But if you are a prospective buyer, you'll still be required to fork out a handsome premium for this stock. Though that's not a move I'd be willing to make at this price, I will definitely keep Facebook on my watchlist.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.