I'm not sure how informal you want to go, but "party animal" is a common term for someone with a busy, even frenetic, social life.
He gained a reputation as a party animal and paid the price for his lifestyle. (independent.co.uk)
It turns out I'm a party animal, and I really enjoy hanging out with students. (guardian.co.uk)
Respected piano-popsmith and Nashville resident Ben Folds isn't much of a party animal. (tennessean.com)
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