To get a feel for the mechanics of co-investing, we asked Sabet (pictured) to walk us through how he met David Karp, the 22-year-old founder of Tumblr Inc., and introduced him to Union Square Ventures co-founder Fred Wilson.

In December, the two firms invested $4.5 million in Tumblr, following up on a $775,000 round the previous year that included TV producer Fred Seibert, entrepreneurs Jakob Lodwick and Martin Varsavsky, as well as Betaworks, an innovative incubator in New York.

To find the next generation of Twitters and Tumblr, last week Spark announced a new seed-stage program called Start@Spark. Through the new program, Spark will invest up to $250,000 each in promising startups in the Northeast.

Click here to watch The Deal's video interview with David Karp about raising Tumblr's Series B. Click here to watch our interview with Union Square Ventures partners Brad Burnham, Albert Wenger and Fred Wilson about Hacking Education.Watch the video interview below or download it at iTunes. -- Mary Kathleen Flynn



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