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

What: Shares of Twitter (NYSE:TWTR) sank 4% this morning after Cantor Fitzgerald downgraded the microblogging giant from hold to sell. Shares have since rebounded from earlier lows, but Twitter is down nearly 20% from its post-IPO highs.

So what: Along with the downgrade, analyst Youssef Squali maintained his price target of $32, representing about 48% worth of downside to yesterday's close. While contrarians might be attracted to the Twitter's recent share-price slump, Squali believes the company's still-lofty valuation and looming lockup expiration will continue to pressure returns.

Now what: According to Cantor, Twitter is a relatively unattractive way to play the social media space at this point. "Putting TWTR's valuation relative to its IPO in context, even compared to other high-flying Internet IPOs, Twitter is in a class of its own," Cantor cautioned. "TWTR's 30.8x EV/Revenue multiple nearly 2 months post-IPO compares to 15x for Google, 16.5x for LinkedIn, 8.5x for Facebook, and 4x for Yelp around the same period." So while it might be tempting to pounce on the stock's recent pullback, I'd agree that holding out for a better price seems prudent.

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.