Amazon ( AMZN) may have enjoyed a revenue hike when it released its second-quarter results Thursday, but investors are underwhelmed by the online retailer's overall performance.

Shares of Amazon plunged $7.60, or 8.10%, to $86.27 Friday, outpacing the broader decline in tech stocks that saw the Nasdaq slip 0.69%.

Despite growing its sales more than 14% year over year, the Seattle, Wash.-based firm posted a mixed bag of results with profit slipping, albeit in line with analysts' estimate. Wall Street had been expecting Amazon to show decent growth, although this was not the case across all of its operations.

The company's global Media business grew just 1% year over year, although Electronics and other General Merchandise (EGM) rose 35%. Falling demand for video games and consoles impacted Amazon's Media division, although this was offset by growth in books.

Amazon's operating income also took a hit during the quarter, plunging 27% year-over-year to $159 million, severely impacted by a $51 million settlement with Toys "R" Us.

Despite its status as something of a tech bellwether, it's worth remembering that Amazon is first and foremost a retailer that happens to use the Internet to sell books, CDs, DVDs and all the other ephemera found on its Web pages.

Amazon, of course, may be the victim of its own success, and has routinely blown past analysts' estimates, so its recent results have been greeted with more of a whimper than a bang.

Rather than seeing Apple's ( AAPL) results as a weak spot for tech, the company's numbers reflect the ongoing unease in the wider economy, which is slowly getting back onto its feet after last year's economic crisis.

Take Amazon's North American Media sales, for example. These were flat year over year vs. increases of 8% in the first quarter of 2009 and 24% in the second quarter of 2008, hardly cause for breaking out the bubbly. At least one analyst, however, thinks that these figures were likely impacted by Amazon's decision to cut prices on its older Kindle e-books.

"We suspect that the sale of lower-priced digital books may be starting to have a cannibalizing effect on the printed material," wrote Youssef Squali, an analyst at Jefferies & Company, in a note released Friday. Kindle sales contributed around 400 basis points to the firm's EGM growth in North America, he added.

Precisely how much Kindle is contributing to Amazon's revenue, however, remains a mystery. Despite the hype surrounding the launch of the firm's Kindle DX, and its Kindle 2 price cut, Amazon still refuses to divulge specific numbers.

Consumers have already cited price as a major downside of Kindle, even after the Kindle 2 price cut, although Amazon insists there is plenty of demand.

"Kindle sales continued to exceed our expectations," said Amazon CFO Tom Szkutak, vaguely, during a conference call Thursday, and was equally fuzzy when asked about unit shipments. "We're not doing any updates in terms of the units, in terms of the units shipped, but we're seeing very, very good growth."

Amazon was also tight-lipped about Web services such as its EC2 and S3 cloud server and storage offerings, refusing to go into specific numbers. Szkutak would only confirm that this part of the company's business is growing "very nicely."

The company's reticence seems somewhat strange, particularly given Amazon's reputation as a trailblazer in cloud computing where users access services via the Internet.

Could it be that, after pushing these technologies for three years, cloud computing remains just a small part of the company's revenue? This would certainly reflect what analysts are saying about the uptake of cloud technology, although Amazon has been beating its Web Services drum recently, forging a partnership with data integration specialist Informatica ( INFA).

The Internet retail giant's results follow good numbers from eBay ( EBAY) and Yahoo! ( YHOO), which both posted decent quarterly figures earlier this week.

Despite its somewhat mixed second-quarter, Amazon is eyeing a better third quarter. The retailer issued relatively bullish revenue guidance after market close Thursday, proving that there is still plenty of upside potential in its future.

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