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 Twitter, Inc. (NYSE:TWTR) slipped 2% this morning after Cantor Fitzgerald downgraded the microblogging giant from buy to hold.

So what: Along with the downgrade, analyst Youssef Squali reiterated his price target of $32, representing about 23% worth of downside to yesterday's close. Although Squali remains bullish on Twitter's growth prospects, he believes that the stock's 60% run-up since its IPO pricing leaves investors vulnerable to a near-term pullback.

Now what: According to Cantor, Facebook (NASDAQ:FB) is the more attractively priced social-media play. As Cantor notes:

While we remain fans of [Twitter] and believe the growth potential is real and significant, we find the current valuation excessive, discounting much of the short-term upside. We move to the sidelines until management begins showing proof of further upside to consensus estimates; we favor FB instead.

With Twitter trading at a clear price-to-sales and EV-to-EBITDA premium to Facebook, which is actually doing a better job right now of monetizing users, it's tough to disagree with Cantor's conclusions.