Third Point's $17 billion stake and its involvement comes on the same day that eBay, under pressure from billionaire activist Carl Icahn, said it will spin off its PayPal payments unit into a separate, publicly traded company. EBay's share price rose 8 percent.
Third Point, a closely watched hedge fund, occasionally pushes companies in which it invests to make management or operational changes. Those companies have included Yahoo and Sotheby's.
At eBay, Loeb pushed the company months after Icahn began loudly calling for eBay to spin off PayPal.
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A source with knowledge of Third Point's interest in eBay did not say what changes the hedge fund sought, or whether it worked in tandem with Icahn.
Loeb declined to comment on Tuesday.
Icahn said on Tuesday he was happy with the spinoff decision, adding it came later than it should have, but sooner than he expected. He said he plans to speak with eBay Donahoe soon.
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Third Point scored another victory on Tuesday, when Masco, which manufactures products for the home improvement and new home construction markets, announced its own spinoff. The hedge fund is one of Masco's top ten investors and had been in contact with the company.