Google missed analyst forecasts for both earnings and income in the third quarter, while the amount it makes when users click on ads also dipped.
Net income dropped to $2.81 billion, from $2.97 billion reported during last year's third quarter.
The company's adjusted earnings per share was $6.35, short of analyst expectations of $6.53, but up from $5.63 in the year-ago period.
Despite the report, company CFO Patrick Pichette spun the news as "another strong performance this quarter". He said: "We continued to be excited about the growth in our advertising and emerging businesses."
Google, however, has faced challenges with advertising on mobile devices, traditionally less lucrative compared to the desktop advertising market. Google does not break out desktop advertising from mobile ad revenue, but declines in its cost-per-click figures have been attributed to mobile.
But Google might be moving toward closing the gap. The average cost-per-click, which includes clicks related to ads served on Google sites in addition to network sites, dropped by 2% compared to last year. However, that's a lower drop than the 7% decline Google reported last quarter.
Paid clicks, which include clicks related to ads served on Google and network sites, rose by roughly 17% during the third quarter, but analysts had forecast them to rise from 22% to 23.5%, depending on the analyst firm making the forecast.
Google's share price took an immediate hit in after-hours trading, plunging sharply from $525.63 just before market close to $498.60, a few minutes after US financial markets closed before leveling off to $524.51 an hour later.