Facebook (NASDAQ:FB) closed the last quarter of 2014 on Wednesday on an excellent note, beating analyst expectations handily. For the full year, Facebook's revenue soared 58% year over year -- faster growth than almost any analyst would have predicted before the year began. For the quarter, Facebook reported revenue and non-GAAP EPS of $3.85 billion and $0.54, up 49% and 69%, respectively, from the year-ago quarter.

Here's a closer look at Facebook's earnings through the lens of three key metrics.

Mobile ad revenue continues to soar
One of the key catalysts for Facebook stock since the company went public has undoubtedly been its roaring growth in mobile revenue. On mobile, Facebook's ads have higher engagement and greater pricing power with advertisers, making the platform more profitable than desktop. Nine quarters after the social network began monetizing mobile, its mobile revenue is still up an impressive 100% from the year-ago quarter.

In Q4, Facebook's mobile ad revenue as a percentage of total ad revenue hit an all-time high of 59%, up considerably from the year-ago proportion of 53%.

Chart is based on data retrieved from SEC filings for quarters shown. 

Ad revenue growth continues to decelerate
While Facebook's year-over-year revenue growth beat expectations, driven by higher than expected growth in advertising revenue, investors should note that the social network's advertising revenue growth rates are decelerating quickly.

Chart is based on data retrieved from SEC filings for quarters shown. 

This trend of decelerating year-over-year advertising revenue growth rates shouldn't be surprising. Monstrous growth isn't sustainable forever -- especially with Facebook now reporting $3.59 billion in advertising revenue in a single quarter. Still, the decelerating growth is a reminder that Facebook doesn't have the same upside for business growth as it did when it first went public.

To be fair, Facebook was subject to meaningfully unfavorable currency fluctuations during the quarter, but even after adjustments, the company's revenue growth rates are still decelerating. Excluding the impact of year-over-year changes in foreign exchange rates, Facebook's revenue from advertising in Q4 would have increased 58%, still down 6 percentage points from year-over-year revenue growth of 64% in Q3.

Engagement finally leveled off
One of the most important metrics to keep an eye on when Facebook reports earnings is engagement, or daily active users as a percentage of monthly active users. If this metric levels off, it could mean the social network is losing one of its important levers for growth.

Going into the report, I pondered whether or not this could be the quarter that the company's engagement metric starts to show signs of leveling off. Indeed, it did. For the first time, engagement remained flat sequentially, sticking to the record high of 64% attained in Q3.

Of course, there's always a possibility that Facebook's engagement metric could resume its upward trend next quarter. Further, even if engagement has leveled off, it shouldn't be a huge surprise; engagement can't trend upward forever. But investors should watch the metric in future quarters to see if engagement actually begins to decline from here.

Overall, Facebook is still firing on all cylinders. Rapid growth in revenue and earnings and high engagement continue to validate the company's excellent business model and captive user base. While investors should expect more deceleration in these unsustainable growth rates going forward, Facebook is still growing meaningfully and executing expertly.