As nouns, these words have the shared meaning of "an informal gathering," "an occasion for conversation and pleasure": "The children are attending a church social (or sociable)." In this use, social is preferred. As adjectives, social and sociable have related meanings, but social is preferred for referring to community living and association in groups: "This is a highly social section of town." Sociable is more often applied to persons than is social in the sense of "friendly," "pleasant," "affable": "Our senator likes people and is a companionable, sociable person." "When you attend that social, try to be more sociable than you usually are."
Dictionary of problem words and expressions. Harry Shaw. 1975.