1) The founders of Mashable, and their stockholders, are earning out on what they've built. That's a good thing, and well deserved.
2) Its really Turner Broadcasting that's doing the buying here. Its an old media company again demonstrating that they are unable, or unwilling, to modernize. They are throwing money at a problem rather than innovating.
3) Such a large amount of money flowing from old media into the technology sector works to hasten the demise of companies like CNN/Turner Broadcasting. It drains their coffers a bit more, while providing new cash flow to the founders and employees in Mashable and frees them up to pursue new innovative ventures over the next few years.
pg really saw something important when he added RFS 9 - Kill Hollywood to the Request for Startups at Ycombinator. While CNN is a "news" company and not directly part of Hollywood, they are most certainly tied at the hip to the same people that run Hollywood. So in that aspect, this is a very good thing.