Facebook shares are surging this year, but one Wall Street firm says recent underperformance is making him revisit his bullish thesis for the internet company.

"If current market trends hold, Facebook's stock price could find itself underperforming the S&P 500 this quarter for the only the third quarter since the start of 2015," MoffettNathanson analyst Michael Nathanson wrote in a note to clients Tuesday entitled "Facebook: Is it Time to Get Out?" "As we approach year end, perhaps with this massive outperformance in mind, we are increasingly noticing a greater degree of skepticism about where Facebook will go in 2018."

Nathanson noted how Facebook shares are up 6 percent since the end of September through Monday versus the S&P 500's 7 percent return. The internet company is up 57 percent year to date compared to the 20 percent gain for the market.

The analyst still reiterated his buy rating and $205 price target for the social media company, representing 13 percent upside to Monday's close.

Smart investors often investigate the opposite thesis to figure out if their current point-of-view is correct.

As a result, Nathanson shared his five top reasons why the company's stock can go lower:

1. "Facebook's Video Strategy Remains a Mystery … we haven't seen much progress" in the last six months.

2. "Will the Next Leg of Growth be Completely Pricing Led? … Facebook has apparently hit the upper bound of its ad load on core Facebook, the story has moved from a largely volume driven one to a largely pricing driven one."

3. "Can It Meaningfully Monetize its Other Platforms? … Despite the significant transaction for Messenger and WhatsApp globally, Facebook's ability to meaningfully monetize them still remains a major question mark."

4. "Emerging Regulatory Risk … a more significant regulatory overhang … could pressure Facebook's multiple."

5. "Everyone Loves … It .. [With Wall Street's] rating distribution that is this overwhelming positive, any hiccups in growth or profitability could lead to a downside reaction that is amplified."

Even with the above reasons, the analyst remains optimistic on Facebook stock for next year.

We believe Wall Street's "top and bottom line estimates for 2018 are too conservative … Facebook remains inexpensive," he wrote.