It makes an interesting case for Facebook's paid messaging service. Combined with some well crafted / personalized messaging, you could probably get a large number to click through. Even if the seed group is a million users, at a dollar per user that's only a million dollars. Large marketers spend that much on advertising without even blinking. If it gave them a reasonable chance at going viral, no doubt many would consider it well worth the investment. The only problem would be the potential backlash when people noticed that it had started in such an artificial way.