If a person is very social in a party, striking up conversations with different people from one end of the hall to the other end, are there some good expressions to describe this person? In Chinese, we say she is a butterfly dancing among the flowers. What is the closest expression or metaphor in English?
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.
Another phrase is "belle of the ball." "Social butterfly" might have a slightly negative connotation in certain contexts. "Belle of the ball" literally means the "the beautiful one at the dance" but is often used to describe a young lady who is socially adept at a particular event and is generally a positive description of such a person.