Google shares slid in extended trading Thursday after the firm posted a profit that missed Wall Street expectations for the quarter, the first reporting period for the company since it named a new CEO.

The Internet advertising giant reported first-quarter earnings of $8.08 a share. Google earned $6.76 a share during the same period a year earlier.

Sales for the most recent quarter rose to $6.5 billion, up from $5.07 billion last year, excluding traffic-acquisition costs.

The company was seen earning $8.10 a share on revenue of $6.32 billion, according to a consensus estimate from Thomson Reuters.

Traffic-acquisition costs (TAC) include money that Web sites pay to advertisers and that they spend to draw traffic. Analysts factor TAC into their earnings estimates.

Google said the average cost-per-click for its search ads in the first quarter increased about 8 percent year-over-year and decreased 1 percent from the fourth quarter.

Shares of Google tumbled more than 4 percent in extended trading Thursday. (Get after-hour quotes for Google here)

The stock closed at 578.51, increased by 2.23 (+0.39%), in the regular New York Stock Exchange session. Volume exceeded 3.2 million shares before the closing bell.

"Clearly the company is still in growth mode and for Google that means spending too—they're spending on stock compensation, on sales and marketing..., really they've hired 1,900 more people this quarter, which might be a new high," said Jordan Rohan, analyst at Stifel Nicolaus. "It's Google being Google, This is what they do."

"This plays into investors' insecurities about how Larry Page is going to run the company," he continued. "Long term, this doesn't change my view of Google. When they feel good, they spend."

In January, Google announced that co-founder Larry Page will take charge as CEO in early April, replacing Eric Schmidt, who had held the top job for the past decade.

Shares of Google, which underperformed the market in 2010, are down roughly 9 percent since the company made the announcement.

Ahead of the earnings release Thursday, Page disclosed the sale of 55,556 shares at prices from $568.30 to $576.85.