At the risk of sounding like a troll, I think the title is unnecessarily specific: yahoo didn't just drop the ball on instant search, photo sharing, social bookmarking, x, or y, or z. They just dropped the ball. Period. As has been extensively documented, Yahoo is a tech company that has never wanted to be a tech company.

Given a few more years, I'm sure they'll get their wish when they cease to be a company altogether.