CEO Mark Zuckerberg has just informed Facebook's staff that his long-restive cofounder, Dustin Moskovitz, is leaving the company. Adding to the blow: Moskovitz, left, is taking with him Justin Rosenstein, right, a top engineer who was one of the first employees Facebook poached from Google as it began its tumultuous rise in 2007. The two are starting a new company together. Rosenstein wrote a much-circulated email to friends explaining why he'd left Google, with the now-famous line, "Facebook really is That company.... I have drunk from the kool-aid, and it is delicious." Rosenstein's note is worth rereading — keeping in mind that, if he's leaving, Facebook must no longer be the company Rosenstein wrote so enthusiastically about:

A couple of months ago, after three years as a Google product manager, I decided to leave for Facebook. I am writing this note to spread Good News to all the friends I haven't already overwhelmed with my enthusiasm: Facebook really is That company. Which company? That one. That company that shows up once in a very long while — the Google of yesterday, the Microsoft of long ago. That company where large numbers of stunningly-brilliant people congregate and feed off each other's genius. That company that's doing with 60 engineers what teams of 600 can't pull off. That company that's on the cusp of Changing The World, that's still small enough where each employee has a huge impact on the organization, where you think about working now and again, and where you know you'll kick yourself in three years if you don't jump on the bandwagon now, even after someone had told you that it was rolling toward the promised land. That company where everyone seems to be having the time of their life. I'm serious. I have drunk from the kool-aid, and it is delicious. Facebook is hiring ambitiously across the organization. If you're an engineer, UI designer, product manager, statistician, bizdev god, general entrepreneurial badass, whatever, and you would even consider considering Facebook as your new place for hat-hanging, please send me a Facebook message. We can have lunch, or I can give you a tour, or we can go kick it with Mark Zuckerberg — whatever it takes.