As with any business, continued growth requires overcoming challenges, and Google (NASDAQ:GOOG) (NASDAQ:GOOGL) is no different. In fact, following its fourth quarter and 2014 annual earnings announcement on Jan. 29th, we learned the challenges Google faces have at least a few shareholders concerned. That said, these hurdles are not necessarily reasons to shun Google as an investment, but investors would be wise to keep an eye on the following key areas.

The song remains the same
Leading up to the latest earnings report, much of the chatter surrounded ongoing problems maintaining its cost-per-click, or CPC, ad rates. Unfortunately, last quarter did nothing to mitigate those concerns. Google CPC rates declined 8% on its sites year-over-year, and that followed a 4% drop the prior quarter.

The problem? As more users access the Internet via mobile devices, advertisers are loathe to pay the same fees as they do for desktop ad results -- and Google is paying the price. Of course, that "price" is somewhat relative. Google continues to set new revenue records year-in and year-out, and last quarter is an indication why. Yes, CPC rates dropped again, but the number of paid clicks jumped an impressive 14% year-over-year, including significant improvement from YouTube.

Another mobile challenge
In addition to the impact mobile device users have on Google CPC fees, increased competition from the likes of Facebook (NASDAQ:FB) is also taking its toll. Of its 1.39 billion monthly average users, an impressive 1.19 billion access the social network via their mobile device. Not surprisingly, all of those Facebook "friends" are accounting for significant mobile ad revenues.

Facebook generated over two-thirds of its $3.59 billion in ad-related revenue from mobile last quarter -- a staggering 53% year-over-year improvement. And this rapid mobile growth is beginning to make an impact on Google's market share. In 2013, Google owned about 50% of the mobile ad market, compared to a 17.5% share for Facebook. Last year, estimates indicate Google market share dropped below 47%, as Facebook jumped to nearly 22%.

Concerning? Sure, declining market share is rarely a good thing. That said, Google is implementing several mobile-specific ad tools for its marketing partners that should help stem the tide. Not to mention, Google remains head-and-shoulders above Facebook and everyone else in digital advertising revenues, mobile or otherwise.

Rumor has it
Google contracts with the manufacturers of mobile devices that are equipped with Google services like search and maps, and that has been a steady source of traffic, helping Google remain the undisputed king of search. However, there may be a significant threat on the horizon -- its long-time agreement with iEverything maker, Apple (NASDAQ:AAPL), as the default search engine on the Safari web browser is soon coming to an end, assuming the rumors prove correct.

Google still owns nearly 75% of all U.S. search traffic, but that is down to levels not seen in seven years. And the Apple iOS and Safari browser carry some weight. In January, about 48% of smartphone Internet searches and 69% of tablet searches were done via a Safari browser.

With numbers like those, the competition is eager to get in on the action, ready to challenge Google for a place as the default Safari search engine. However, it will be no easy task to dethrone the reigning king of search.

Like the falling CPC rates and mobile challenges, Google will need to address this declining market share. Overall, the company remains a strong investment option for mid and long-term investors.

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.