Google's core business is slowing down

Google's total advertising revenue was $15.5 billion last quarter, up 11% from the same period of last year, it said in its earnings Thursday. The number of clicks on ads ("paid clicks") was also up 13% year-over-year.

But it's not all good news for Google, whose advertising revenue accounts for nearly 90% of its total sales.

According to BI Intelligence, Google's ad sales growth has been on a steady decline over the last few years. Just 3 years ago, it was growing more than 20%, but by last year, the growth dipped to around 15%.

Google's cost per click (CPC), or how much it can charge its ads, has also dropped by another 7% last quarter, which hints the company could possibly be having trouble with its most important business.

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