The eBay and StumbleUpon union stumbled on Monday after the social networking site's founders, Garrett Camp and Geoff Smith, said they bought back the company with a group of investors, including Accel Partners and August Capital.

EBay Inc, which acquired the fast-growing company in May 2007 for $75 million under the tenure of eBay's former Chief Executive Meg Whitman, would not disclose the price.

We didn't find synergies with the eBay portfolio, said eBay spokesman Usher Lieberman.

Founded in 2001, StumbleUpon, which now has more than 7.4 million users, lets consumers use a thumbs up/thumbs down system to rate websites and videos, allowing users to personalize interesting links on the Web.

EBay has been focused on reinvigorating growth in its core marketplaces business, which includes auctions. EBay has acknowledged that its Web telephone company, Skype, also acquired under Whitman's tenure, has no synergies with its main business, and many on Wall Street believe a sale is imminent.

(Reporting by Alexandria Sage; Editing by Andre Grenon)