It's probably an artifact of the data that Wealthfront is collecting. They're catering to people with wealth to manage, and critically, they omit founder-level employees in this chart. If you look at the people who've made over, say, $5 million from equity in Silicon Valley in the last couple of years, there are probably many more Facebook employees with 0.01% stakes than there are 1% employees at companies acquired for $500 million, or 2% employees at companies acquired for $250 million (above that and you're probably a founder).