David Karp doesn't look like a Champagne-swilling clubgoer who parties with P. Diddy. Tall and reedy, the young techie—he turned 27 last week—looks like he'd be more comfortable in front of a computer monitor than under a strobe light.

But in late June, the Tumblr CEO had cause to celebrate, waving a bottle of bubbly in the air amid the thrum in a Cannes discothèque in widely circulated photos. Mr. Karp had recently sold his company Tumblr, a blogging service, to Yahoo for $1.1 billion. (His estimated take: $250 million.)...