Someone who has money and doesn't work, instead devoting his/her life to being "socially active." Socialites go to parties, gather media attention, and essentially "work" at being popular. This often comes at the expense of any meaningful contribution to society or culture (outside of catchphrases-that-aren't-really-catchphrases, like Paris Hilton's "That's Hot").
A person who is rich and pretty.
Examples: Marilyn Monroe, Paris Hilton, Kim Kardashian
Camilla: Omg, we are so pretty!
Jasmina: Ikr. That's why we're the socialites!
Emma: Marilyn Monroe has nothing on us because she's ugly and we're pretty!
Helena: Hey girls, did you get the latest Playboy issue? Because I was in it!