Considering that Hackermeter didn't really have a chance to build our their Product or their Business, Pintrest was probably interested in the Founders Personally (as in, their Skillset). It seems that some people view this entire scenario with a negative light, but I think they did a great job. They got into Y-combinator, they launched their Product, and they were getting Traction. That right there is no easy task, regardless of what I think (or understand) about their Product. Pintrest probably saw them as being valuable, and wanted them on the Team. Good for them! The only downside here (from what I can see) are all the people that signed up and invested time doing these challenges, to then have the service shut down and have nothing to show for it. That's probably pretty frustrating as an End User.
Acquihire. The product has been scuttled.
I believe YC accepted them for the idea and not for their strategy for acquihire. Kind of baffles me that just a 2 month old company gets acquired for talent, and that too for a not-an-out-of-the-world product
I assume they weren't able to get any additional funding, and were at the point where they needed to wind down the company. Another company comes along and says "we'll give you good jobs, and make your investors whole." So they take it. It's not like there was a Plan B.