Ever wonder how Twitter CEO Jack Dorsey spends his Christmas vacation? Neither did we ... but we spotted him in St. Barth with his model girlfriend so now we know!

The bearded billionaire wore Rick Owens drop-crotch shorts and artisanal handmade leather sandals. His Sports Illustrated model girlfriend Raven Cornell wore a bikini with white boy shorts and a Chanel butt pack.

The pair are cruising around on his Yacht, Getaway, in the French West Indies where Dorsey is known to spend time with beautiful, young women ... nothing wrong with that!

We do have one question, though ... with a net worth estimated at $4.4 billion, why hasn't Dorsey signed Warren Buffet's Giving Pledge? Even if he gives away half his wealth -- like Bill and Melinda Gates have promised, and others like Buffet -- he's still have plenty of money for his yacht and his vacation and Chanel butt packs ...