Could Carl Icahn be losing his magic touch?

John Jannarone

For a few moments, Wednesday looked like a typical day for activist investor Carl Icahn: Take a stake in a company and watch the shares fly as soon as the public becomes aware of the move.

Shares of eBay rose as much as 10 percent, to about $60, in after-hours trading Wednesday after the company announced that Icahn had taken a 0.82 percent stake. Icahn had an idea for unlocking value in the online marketplace business: spinning off the faster-growing PayPal operation.

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But the stock began to fall before long and by Thursday morning had lost most of its gains, trading up just 0.75 percent at $54.80. The trouble was that Icahn's suggestion was probably less feasible than some of his typical proposals.

Carl Icahn, chairman of Icahn Enterprises Holdings
Scott Eelis | Bloomberg | Getty Images

Consider his more common tactic of taking a stake in a company, then suggesting another company should acquire it to take advantage of big synergies. That was his approach with Clorox and Mentor Graphics, whose shares surged after his stake was announced. While neither company was bought, the stocks stayed high for days or weeks on hopes that a buyer would emerge.

But there's much less mystery in eBay's case. The company said on an investor call Wednesday night that it has already considered a split and laid out several credible reasons that it doesn't make sense.

Icahn didn't respond to a phone call from

First, eBay's marketplace business helps PayPal pick up new customers with no marketing costs. Second, PayPal collects data about individuals that help the marketplace underwrite risk.

Further, eBay's marketplace generates plenty of cash, which allows PayPal to concentrate on gaining market share without being overly concerned about margins. And with customers migrating to mobile from desktop, it's a dangerous time for eBay to dismantle a business model and risk losing business to rivals like Amazon.

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Ebay makes PayPal better: Donahoe

Icahn's plan could have validity if there were a way to create two stocks without interfering with the relationship between eBay's marketplace and PayPal.

Wells Fargo analyst Matt Nemer pointed out that media mogul John Malone has unlocked value by creating "tracking stocks" that give investors exposure to separate companies without breaking up the businesses. Indeed, PayPal should probably trade at a higher multiple than eBay given its faster growth.

And in fairness to Icahn, eBay shares likely would have been even weaker if he hadn't shown up. The company issued fourth-quarter results Wednesday evening that included a lower earnings forecast for 2014.

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Icahn has been successful over his long career by taking positions in companies and asking for change. But with so many impediments to his eBay proposal and little hope of another bidder buying the entire company, investors should shop elsewhere.

—By CNBC's John Jannarone. Follow him on Twitter @jannarone.