????一年前，鮮有投資者看漲Facebook。有些文章的作者甚至認為，23美元的股價仍然有水分。其他人則認為，瑪麗莎?梅耶爾執掌的雅虎（Yahoo）更有希望扭轉不利局面。此后雖然有些看跌者開始緩和悲觀看法，但普遍觀點是，Facebook估值過高，具有投機性。

????過去的一年中，Facebook股價漲了115%，而納斯達克綜合指數僅上漲了37%。市場調研公司Thomson/First Call跟蹤的40位證券分析師中，目前有32位給予這只股票“買入”或“強力買入”評級，8位分析師給予“持有”評級，沒有一位給予“賣出”評級。即便大幅反彈之后，目前該股的空頭頭寸只有一年前的1/3。

????到底是什么發生了變化？造成Facebook股價大逆轉的主要原因是它移動廣告的突然增長。當初，這只股票的股價還在20多美元區間內徘徊時，Facebook就瞄準了受眾從桌面電腦轉向移動設備的趨勢，準備從中掘金。它聚焦移動的努力獲得了成效。一年前，它的收入增長率只有32%。截至9月份的最近季度，它的收入增長了60%。

????但最近幾周，有些跡象表明，Facebook的大幅反彈開始失去了動力。上周，這只股票曾下探到45.73美元，較1個月前創下的54.82美元紀錄高點下跌了17%。本周一，它在47美元上下交易。這是下跌的前期信號，還是短暫走弱？展望未來，值得考慮的問題是，什么因素可能結束Facebook這一輪反彈。

????Facebook的業務改善已體現在股價中。這家公司的復蘇十分顯著，導致人們對它的期望值高得有些離譜，甚至比它去年5月上市的時候還要高。這些使得Facebook的估值非常高，就連未來幾年尚未發生的增長也已經計入股價。Facebook當前股價是它過去12個月每股收益的125倍，是2014年預計每股收益的44倍。

????還有其它幾個因素將推動Facebook增長：廣告的受眾投放更加準確，為廣告主提供更佳回報；將開始在Instagram和Facebook視頻上賣出更多廣告；能建立一個更強大的第三方廣告網絡；廣告主對Facebook廣告的需求當前沒有顯現出任何放緩的跡象。這些是樂觀的預期，但這也是眾所周知的事情，不能為這只股票繼續走高提供支撐。

????Facebook將再次出現增長率下滑的現象。分析人士預測，Facebook 公司2013年收入增長50%，2014年增長36%。這還是在上述所有的增長動力推動之下。但所有這些都取決于一個答案非常不確定的重要問題：Facebook還能向移動端注入多少廣告、同時又不會把用戶趕跑？

????Facebook首席財務官戴維?埃伯斯曼在上次業績電話會議中表示，公司“顯著”增加了電腦版廣告，“適度”增加了移動端廣告。它計劃將廣告內容保持在移動端信息流內容的5%左右，依靠使用量增長和需求增加來保持增長。

????A year ago, few investors were bullish on Facebook (FB). Some writers argued that, at $23 a share, it was still overvalued. Others thought Yahoo (YHOO) under Marissa Mayer had a better shot at a turnaround. While some bears were beginning to temper their pessimism, the consensus view was that Facebook was expensive and speculative.

????In the past year, Facebook's stock has risen 115%, against a 37% rise in the Nasdaq Composite Index. Of 40 securities analysts tracked by Thomson/First Call, 32 currently have a buy or strong buy rating, 8 have a hold rating and none have a sell rating. Even with the rally, short interest on the stock is a third of what it was a year ago.

????What changed? The main reason for this reversal of fortune in Facebook's stock is the company's sudden growth in mobile ads. Back when the stock was languishing in the $20 range, Facebook vowed to monetize its audience's shift from desktop computers to mobile devices. That focus on mobile worked. A year ago, revenue was growing at only 32%. In its most recent quarter September, revenue grew 60%.

????In the last few weeks, however, there have been a few signs that Facebook's impressive rally is running out of steam. Last week, the stock traded as low as $45.73, or 17% down from its record high of $54.82 one month ago. The stock was trading around $47 a share Monday. Is this decline an early sign of a downturn or just a quick sputter? Looking ahead, it's worth considering factors that could derail the Facebook rally.

????Facebook's turnaround is priced into its stock. The company's recovery was so impressive it paradoxically set the bar of expectations higher even than it was when Facebook went public in May of last year. That has left Facebook very expensive, priced for growth that won't happen for a couple of years: Facebook is trading at 125 times its trailing 12-month earnings, and at 44 times its estimated earnings in 2014.

????There are still several factors that will drive Facebook's growth: the company is better targeting its ads to deliver stronger returns for advertisers; it will start to sell more ads on Instagram and on Facebook videos; it can build a stronger third-party ad network; and advertiser demand for Facebook ads is showing no signs of slowing right now. That's a sunny forecast, but it's well known. It doesn't justify the stock moving higher from here.

????Facebook will see growth rates decline again. Analysts are forecasting 50% growth in Facebook revenue for 2013 and 36% for 2014. That's with all of the growth initatives listed above. But all of this hinges on an important question with a very uncertain answer: How many more ads can Facebook load into its mobile feeds without driving away users?

????In Facebook's last earnings call, CFO David Ebersman said the company "significantly" increased desktop ads but "modestly" increased those in mobile feeds. The company plans on keeping ads at around 5% of mobile-feed content, relying on growing usage and rising demand to maintain growth.