Lauren Santo Domingo is giving the job—yes, “job” because she makes it pay—of socialite in New York City some 21st-century updates. Sure, she’s got icy-blond good looks, a millionaire upbringing and a billion-heir husband, but she’s also managed to make bank on something she’s an expert at: exclusivity. In 2011, Ms. Santo Domingo co-founded Moda Operandi, a fashion startup that allows members to pre-order looks straight off the runway, months before they hit stores. Public-embarrassment- and reality-show-free, Ms. Santo Domingo (with credit to her PR past) has also managed to bring Gilded Age respectability back to the “Manhattan socialite” moniker while still embracing (and properly using) social media platforms. Ms. Santo Domingo is a pro at keeping it classy, and her hydrogen peroxide-sharp tweets (followed by 58,000 users) and envy-inducing Instagram feed prove she’s a socialite for the digital age.