There are only two countries in the world that tax non-residents: America and Eritrea.

http://renunciationguide.com/Citizenship-Based-Taxation-Inte...

As there is no representative for American citizens abroad (they can vote in Federal elections, but the district is based on place of last residence), this is essentially taxation without representation.

Saverin has lived in Singapore since 2009. He renounced his citizenship in 2011. http://en.wikipedia.org/wiki/Eduardo_Saverin

Given the situation, I don't see why he would keep American citizenship.