After posting another solid quarter, and year, you'd think Facebook (NASDAQ:FB) stock would be flying high. In fact, just recently, several analysts have either maintained or even raised their price targets, including one suggesting Facebook will hit $105 a share. Considering Facebook is currently sitting at about $74 a share and change, that's a lot of upside, especially after performing so well in 2014.

But based on Facebook's stock price since announcing earnings on Jan. 28 -- after an initial pop, it's actually declined by over $3 a share -- investors have a few concerns. And while Facebook does face some challenges, that doesn't necessarily mean its recent decline in value is warranted, or that it will continue. However, for investors considering Facebook, there are a few areas worth keeping an eye on.

The elephant in the room

Though Facebook CFO Dave Wehner warned investors months ago that spending would increase, this recent quarter's expenses still caught a few by surprise. As per Wehner before the recent earning news, overhead could jump by as much as 70% compared to previous quarters. Turns out, Wehner was overly optimistic.

On a GAAP (including one-time items) basis, Facebook spent a whopping 87% more in 2014's Q4 than it did the prior year. The dramatic increase in overhead was attributed to a few factors: as per COO Sheryl Sandberg, Facebook continues to invest in its ad measurement tools, as well as prepping for the roll-out of video spots across its site.

Toss in infrastructure-related costs and assimilating recent acquisitions, particularly the $19 billion WhatsApp, let alone the $2 billion deal for Oculus, and overhead was sure to skyrocket. Worth keeping an eye on going forward are future expenditures. Why? Because according to Wehner, Facebook is likely to increase spending even further in 2015, by as much as 50% to 75% over last year's inflated outlay.

A dose of Instagram, if you please

As noted in a recent article leading up to Jan. 28's earnings news, there were a couple of key areas Facebook CEO Mark Zuckerberg could share that would have investors cheering. One, pushing high-priced video ads out to the masses will begin paying dividends in 2015.

Facebook video ads are coming none too soon as it tries to make a dent in Google's (NASDAQ:GOOG) (NASDAQ:GOOGL) dominant video ad market share, thanks to its wildly popular YouTube property. According to eMarketer estimates, video spots will generate nearly $8 billion in sales this year, of which Google owns about 19%. And Google's ramping up its own video spot solutions, to not only gain market share, but fend off the likes of Facebook.

The other item investors would have loved to hear was that Facebook was making a concerted effort to truly monetize Instagram -- now. Unfortunately, that was conspicuously absent from Facebook's earnings call. There's no denying Instagram and its now over 300 million monthly average users (MAUs) is ideally suited for Facebook-like spots, especially video -- it's a natural for a photo-sharing site. But it appears investors will need to wait a while longer.

As for the future

Though certainly not a major issue, another area worth monitoring is Facebook's MAU growth -- or lack thereof. Last quarter's 1.39 billion MAUs was only a slight improvement over Q3's 1.35 billion. Of course, Facebook properties like WhatsApp boasts over 700 million MAUs, and Messenger has skyrocketed to over 500 million, so considering its multiple platforms, user growth continues. However, as things stand, its primary site is responsible for the vast majority of revenues today, so until its other platforms start adding meaningfully to sales, growth hinges on Facebook alone.

The MAU "problem" is that there are more people across the globe without Internet access than with it. So until Zuckerberg's Internet.org initiative takes off, and Google starts sending its Project Loon connectivity balloons and solar-powered drones into orbit, where's Facebook's MAU growth going to come from?

When it's all said and done, analyst estimates of $100 a share aren't far-fetched as Facebook continues pumping up the ad revenues. But as is the case with any stock, that doesn't mean there aren't a few roadblocks Facebook will need to overcome along the way. Are expenses, delays in monetizing Instagram, or slowing MAU growth deal-breakers? No, not at all, but they are worth keeping tabs on.