@0x3F "social interaction networks" sounds cool.
It will also be a lot more kickass abbreviation than "social media".
@0x3F well, "media" is a Latin word, the plural of "medium", which means more or less "between", like a bridge is between two rivers. As always, the current meaning depends on the original one: by staying between, the "media" let people communicate.
Nothing related to marketing, in itself. The fact that "social media" became synonymous with "marketing platform" is a degenerate effect of marketing itself: it's change the meaning of words to make some concept unthinkable.
1984, is now! ;-)