Earlier today, I noted how venture capitalists are struggling to find positive returns in this tough economy. But it's not just the VCs that are hurting.

Amazon.com today said that the value of its investments in private companies fell to $89 million in the first quarter, down from $247 million at the end of last September. That's a whopping 64 percent decrease in six months. The online retailer has been one of the more active investors in new Internet startups in recent years, taking stakes in companies such as Yieldex, Wikia, Elastra and Engine Yard.

Earlier this month, Amazon invested in Seattle online recipe site Foodista and San Francisco startup BookTour.

But just like venture capitalists, Amazon is having a hard time figuring out what value to place on startup companies these days.

"The current global economic climate provides additional uncertainty," the company wrote in today's SEC filing. "Valuations of private companies are inherently more difficult due to the lack of readily available market data. As such, we believe that market sensitivities are not practicable."

Amazon.com was burned badly during the last downturn because of its invesments in failed Internet companies like HomeGrocer.com, Kozmo.com and Living.com.

However, there's a big difference this time around. Unlike the multi-million dollar bets at the turn of the last decade, Amazon is now putting far less capital to work in individual deals. For example, both Foodista and BookTour raised less than $600,000.

If the startup companies are unsuccessful, the giant online retailer should be able to stomach those types of losses. However, no matter how small the loss, investors may begin to question why Amazon.com continues to bankroll these money-losing ventures.

[Story via paidContent.org]

UPDATE: R. Scott Tilghman, an analyst at Hudson Square Research, notes in an email that some of the declines in Amazon.com's private portfolio were tied to the sale of its stake in Bill Me Later to eBay. Since Amazon's investment would have been cashed out, Tilghman said the portfolio valuation would have been reduced. The Venture Capital Dispatch blog also notes the possible effect of the Bill Me Later sale.