This One Simple Metric Is (Probably) Holding Facebook Stock Below $55

One Wall Street analyst expects Facebook stock to hit $72.50, but the price remains around $54.

On average, Wall Street analysts are expecting Facebook to eventually hit $58, but for weeks now FB has failed sustain a rally over $55.

And we're all about to start hearing a lot of yelling about Facebook's business "slowing down."

There is one metric that's causing all the fuss: comparables.

More specifically, tough comparables: In the finance world, analysts compare this year's number to the same number last year at the same time period - as opposed to the most recent previous number from one quarter ago. This helps them figure out how companies are growing year to year, and it knocks out any seasonal bias from the holiday shopping seasons of summer and winter.

But comparables also distort a company's business when, for instance, a year-old line of revenues are being compared to the year-before period, when they didn't exist.

Facebook has previously benefitted from such a distortion, but in Q4 it will get the other end of the stick:

Back in Q2, Facebook's stock got an enormous boost when investors belatedly realized that the company's mobile ad business was turning out to be huge. Facebook benefited from the fact that its mobile ad revenue number was being compared to the Q2 2012 number - a time period when Facebook didn't have any mobile ads.

Then, in Q3 2013, the mobile ad revenue number - about $1 billion - was initially welcomed, but investors became more cautious when CFO David Ebersman began talking about a loss of teen activity on the site during the conference call with analysts.

Facebook, of course, only began its mobile ad program in earnest back in Q3 2012.

So Facebook's Q4 2013 earnings - and specifically its mobile ad business - will be compared to its Q4 2012 number, a time period when mobile ad revenue was up and running nicely at Facebook. Facebook had about $364 million in mobile ads that quarter.

The Q4 number will still be a huge increase in mobile ad revenue, but the percentage increase will be smaller.

So expect to hear a lot of screaming about "Facebook's mobile ad business is slowing down!"

It's probably not slowing down, it's just that the comparables are tougher.

Disclosure: The author owns Facebook stock.