Facebook Inc. (FB) shares are handily outperforming the broader market this year, having risen 55% compared to the S&P 500’s mere, albeit still strong, 20% rise. But the stock's performance has lagged in the fourth quarter, which is unusual in recent years. And that, along with other factors, has fueled rising skepticism among investors over whether Facebook can continue to outperform the stock market in 2018.

Rising Skepticism

While maintaining their buy recommendation for the stock, analysts at the research firm Moffett-Nathanson believe that "the Facebook bear case will only continue to gain momentum," according to Barron’s. Moffett-Nathanson outlines a lengthy list of concerns about Facebook even though the social media site has grown into a giant that boasts 6 million advertisers and 2.1 billion monthly users representing 58% of the world’s Internet users, a number that is growing.

Here is a look at the firm's concerns about Facebook from its 25-page report, per Barron's:

The Bear Case

*Growth Challenges. Facebook has apparently hit "the upper bound of its ad load on core Facebook" and has moved largely from a volume-driven model to a price-driven one, the report says, per Barron's. This may require Facebook to look for new areas to get large growth in ad revenue.

*Flawed Video Strategy. One of the social network's biggest problems has been its video-content strategy, or lack thereof. Facebook's engagement among younger demographics is on the wane. As a result, improving Facebook's video product is crucial "if it wants to stave off further engagement declines," the report says, per Barron's.

*Weak Monetizing of Messenger and WhatsApp. While both apps now have strong global presence, Facebook has not yet figured out how to “meaningfully monetize” them, according to the analysts. (To read more, see: How WhatsApp Makes Money).

*Costly Regulatory Investigations. The recent congressional investigations into Russian-linked ads on Facebook could be extremely costly for the social network, eating up "years of incremental investment," the report says, per Barron's.

*Big Downside Risk. With more than 80% of analysts posting a buy rating on Facebook, Moffett-Nathanson says "any hiccups in growth or profitability could lead to a downside that is amplified."

To be sure, Moffett-Nathanson points to a long list of factors that also support the bull case for Facebook in the coming years, including continuing growth in monthly active users, rapid growth in developing markets and its trove of data that's coveted by marketers - along with its huge scale.

But even with this, Moffett-Nathanson sees enough weakness in Facebook to give the firm pause, enough to make the "bear case" a viable scenario for the world's most successful social network. (To read more, see: Google and Facebook's Growing Ad Dominance Calls for Caution: Pivotal).