Facebook is my choice.
World rankings are as below for those sites as per Alexa rankings. This would help you to get an idea how popular those sites among world's internet users and US's users.
Facebook (World Rank -2 , US rank - 2)
Twitter (World Rank -12 , US rank - 12)
Hi5 (World Rank - 56 , US rank - 413)
Orkut (World Rank -57 , US rank - 591)