What is the best way to describe someone who is very social in a party?
An almost exact English equivalent is social butterfly.
Note how the etymology mirrors the Chinese expression:
Definition: a very sociable person who flits from one social event to another
Etymology: likened to a butterfly flitting between flowers
You could also say (s)he's the life of the party
They could be described as gregarious.
http://en.wiktionary.org/wiki/gregarious