Google's revenue increases 20 percent, but below Street view

(Reuters) - Google Inc's (GOOGL.O) (GOOG.O) third-quarter revenue grew 20 percent, but slightly below Wall Street expectations, as growth slowed in the Internet company's total number of money-making ads and expenses increased.

Shares of Google were down 2.7 percent at $510.11 in extended trading on Thursday.

Google posted $16.52 billion in revenue for the three months ended Sept. 30, compared to $13.75 billion in the year ago period. Analysts polled by Thomson Reuters I/B/E/S were looking for revenue of $16.57 billion in the latest quarter.

Google's total number of ads, or its paid clicks, expanded by 17 percent year-on-year in the third quarter. That was down from the 25 percent growth rate that Google delivered in the second quarter.

Google's online advertising rates, which have been in a multi-year decline, moderated slightly in the third quarter, declining 2 percent year-on-year, compared to the 6 percent decline posted in the second quarter.