High-flying startup StumbleUpon has been rumored to be in acquisition discussions since at least last November. Recently we’ve heard that talks have heated up again, with Google, AOL and eBay as potential suitors. A source with knowledge of the deal now says the company has signed a term sheet with eBay to be acquired. The price is somewhere between $40 - $75 million. (update: GigaOm is now reporting the price at a $40 - $45 million).

StumbleUpon lets users rate websites via a browser toolbar. At any time a user can click “Stumble!” and will be taken to a website highly rated by other StumbleUpon users who tend to vote in a similar way as the person “stumbling.” More often than not, it’s something almost serendipitously interesting to the reader. The company expanded into video referrals in late 2006.

People who are passionate about StumbleUpon say they like it because of the surprise factor in what they see next, and the fact that the product has such a high hit rate in delivering interesting new content. The StumbleUpon site says they have 2.1 million users, up from 1.7 million in December 2006. 4+ million sites are “stumbled” daily.

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