Danielle Morrill just published thoughtful post about Zombie Startups, namely her own, Referly, falling into that zone. Via one of her investors:

“The biggest problem we see with early stage companies coming out of YC, or really any program, is that they’ll approach a year or two after they’ve graduated to raise a seed round. It’s exciting to see they’re still alive and pursuing their vision, but then we ask about the growth of the team and the ways they’ve been capturing the opportunity of the business in the time they’ve had… and discover everything is the same. The same 2 or 3 people, the exact same idea, very little growth around key metrics like engagement or revenue. So why should try raise a series A? What have they proved?”