Google's social network site Google Plus just started using another social network: Twitter. Mark Blinch/Reuters

Google disappointed investors for the seventh time in nine quarters Thursday, posting earnings that slightly missed analysts' estimates for the first quarter of 2015 as its growth continues to slow in its key search advertising business.

The Mountain View, California, search giant posted earnings of $6.57 per share with revenues of $17.3 billion, up 12 percent from the same period last year. That looks impressive, but unfortunately for the company, those figures missed analysts' estimates of $17.5 billion for revenues and earnings of $6.60 per share. The 12 percent growth is also drastically down compared to the 19 percent Google grew between the first quarters of 2013 and 2014 -- and will give investors more reason to worry about whether the immensely powerful company has finally reached its peak.

Like Facebook on Wednesday, Google blamed its disappointing revenue on wild fluctuations in foreign currencies, saying it would have posted an additional $795 million in revenue had exchange rates remained constant over the past year. “Excluding the net impact of foreign currency headwinds, revenue grew a healthy 17 percent year on year,” said Patrick Pichette, Google's chief financial officer, in a statement.

Google doesn't break out its revenue streams to show how much each of its businesses contributes, but many experts agree that the company has been unable to dominate the mobile advertising market the same way it does with desktop. According to eMarketer, Google saw its slice of the global mobile ad market shrink from 46 percent in 2013 to 38.2 percent last year while rival Facebook boosted its share from 16.4 percent to 17.4 percent during the same period.

The biggest issue for the company is the continued drop in its cost-per-clicks, which is how much Google is paid every time users click on an ad. The cost-per-click on Google websites decreased 13 percent year to year while aggregate cost-per-click, which includes ads on third-party websites, fell 7 percent in the same period. That's an indication that demand for Google ads is not what it once was.

"Their struggle is that a lot of people are shifting their behavior to mobile. They're going into apps, and I feel [Google] isn't able to reach those people in apps," said Martin Utreras, senior forecasting analyst for eMarketer. Utreras said this is why Google recently adjusted its search algorithmns to prioritize mobile-friendly websites. "They're losing some of that audience there."

With Thursday's results, concerns will grow over how well-positioned Google is to bounce back in the smartphone advertising market.