• by Joe Mandese  @mp_joemandese, October 3, 2013

While remaining “very positive” on its fundamentals, ad-savvy Wall Street analyst Brian Wieser this morning downgraded shares of Facebook stock from a “Buy” to a “Hold” recommendation due to the overzealous optimism of investors on its revenue expectations.

Despite maintaining its recent $48 target price for Facebook shares, the Pivotal Research Group analyst writes that “the market has over-corrected to the upside” following some “undue bearishness” that preceded strong second-quarter earnings.

“That optimism may persist ahead of earnings, only to be let down if expectations begin to circularly run further ahead of themselves,” Wieser said in a report sent to investors this morning.

The downgrade follows a series of upgrades since Facebook went public -- from “Sell” to “Hold” to “Buy” and back down to “Hold” again.

Noting that his model still projects a 55% increase in Facebook's advertising revenues during the third quarter -- a slower rate of growth than the 61.2% gain in the second quarter -- due to “tougher comparables,” Wieser nonetheless said it is “difficult to get excited about the stock from here.”

Among other things, Wieser said he is not as exuberant as investors about the prospects for incremental revenue growth for Facebook in some of the most dynamic areas of digital advertising, especially video and mobile.

“For example, on video advertising, our view is that Facebook will not capture meaningful budgets that would have gone to traditional TV,” Wieser concludes, adding,” Similarly, on mobile advertising, Facebook has seemingly been elevated on gains in this field, too, despite the fact that Facebook generally allocates budgets it captures to different devices based on its algorithms; the company does not necessarily capture much incremental revenue that is purely about mobile, per se.”