While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a look at particularly stock-shaking analyst upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of Facebook, Inc. (NASDAQ:FB) slipped 1% on Monday after Argus downgraded the social-networking gorilla from buy to hold.

So what: Along with the downgrade, analyst Joseph Bonner removed his price target of $71, an indication that he sees limited upside for Facebook shares in 2014. So while momentum traders might be attracted the stock's strength in recent months, Bonner's call suggests growing sentiment on Wall Street that the valuation is becoming a bit stretched.

Now what: According to Argus, Facebook's near-term risk/reward trade-off is pretty balanced at this point. "[W]e believe that management is again emphasizing long term growth in the user base over the immediate monetization of existing services," said Bonner. "This shift in emphasis is exemplified by the company's plan to shelter WhatsApp in order to build the user base, rather than quickly monetizing the service in the manner of some competitors. We are concerned about the impact of this strategy shift and the risks of the WhatsApp acquisition." Given Argus' still-very bullish view of Facebook's five-year appreciation potential, however, long-term investors would do well to take today's downgrade with a grain of salt.

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.