Interestingly enough the Fed recently disclosed one of the founders gave up his US citizenship in 2011 in order to minimize the exit tax on his 3.2 billion FB holdings.
That is a smart move on his part. He moved here as a child, went to private school and Harvard and somewhere along the line became a US citizen, was involved in a startup, got a share but not a role. He has no ties to the US and doesn't live here, so why stay?