That's not a fair comparison. Facebook's mafia guys are mostly early-stage employees. Early-stage employees tend to be more entrepreneurial and are also more tight-knit, which is how the mafia sentiment is able to sustain itself.

Plenty of Googlers are entrepreneurial enough to leave and do startups, but they aren't considered mafia because they were never part of an early core group that bonded. And those that are entrepreneurial enough to leave aren't as high-profile entrepreneurial as early-stage employees would be.