The thing is that you’re also assuming there’s taxes to be paid on the money he’s making. He paid all of the taxes on Facebook’s IPO offering now, on what he’s worth. That’s a part of the process of renouncing US citizenship. Also, Bloomberg estimated by renouncing citizenship, Saverin is only saving about $67M. Not billions (he’s “only” worth about $2.5-3B). Is that significant? Sure, it’s more than either of us are making. But think about the companies that move off-shore every year and the money they’re no longer giving to the country. That’s a bit more significant. The US still allows them to do business here as well.
Also, Singapore does not allow dual-citizenship, which is another reason he’d need to renounce his US citizenship.
The US is also the only major country that taxes based on citizenship rather than residency (And one of two period, with Eritrea doing it as well), . Meaning that if you were a US citizen living in another country, you still pay US taxes (as well as the taxes to that country, I assume).