The most notable example of social inequity in The Great Gatsby by F. Scott Fitzgerald is that between the old money elite (as represented by the Buchanans) and the new money denizens of West Egg (as represented by Jay Gatsby). Although the blue-bloods will happily visit Gatsby's parties, they will never accept him as one of their own.
Gatsby may be fantastically rich, live in a massive house, and enjoy all the trappings of wealth. But to the old money elite, he's nothing but a "parvenu," a French word for someone who has not rightfully earned a higher level of social class.
This snobbery towards Gatsby helps explain Daisy's inability to leave her husband Tom, despite his being a brute and serial philanderer. Daisy feels more comfortable with the old money establishment that not even Jay, with all his enormous wealth, can buy his way into.