Yesterday’s Q3 earnings report and the following investor call offered us some precious glimpses into Google’s operation. We learned, for instance, that we should not expect a pure Google phone from Motorola in the following six months.

Here’s another fascinating tidbit: Google’s mobile business is growing like Topsy. The company’s mobile run rate in the third quarter was an impressive $8 billion, more than three times the run rate from the same quarter in 2011, which was $2.5 billion. Does this mean that Google managed to triple its mobile business in just one year? Not exactly. Let’s see why.

First, what is a run rate? In investor talk, the run rate is a way of extrapolating future performance based on a current performance. For instance, if a company obtains revenues of $1 billion in Q1 2012, the run rate would be $1 billion x 4 (the number of quarters). Obviously, the run rate can be very misleading – if a company has a blowout quarter (due to the holidays, for example), the run rate would be much higher than the actual revenues that the company would obtain in the rest of the year.

So the $8 billion figure that Larry Page flaunted on the earnings call could be misleading. But there’s another thing to know – Google specified that the mobile run rate now includes the revenue that come from the Play Store (apps, movies, music, etc.), and not just revenue from mobile ads (like it was the case in Q3 2011).

Google’s mobile business is blossoming, but revenue from mobile ads, the main focus of Larry Page and Co, is not growing as fast as the $8 billion run rate would let you believe. And this relative slow growth explains the 20% percent drop in overall profit that sent shock waves through the market yesterday.

Google’s CFO Patrick Pichette went on to clarify that mobile ads do make up for most of that $8 billion, but he refused to be more specific.

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