It seems people could unfriend each other as early as in the times of Shakespeare and even before:
"The word ‘unfriend’ is, like the word ‘muggle’, one that has a curious history: ‘unfriend’ had a life before Facebook. Its principal meaning now is to delete somebody as a friend on a social media side, especially Facebook, but it has been used variously as both a noun and verb since at least the thirteenth century. Its origins are somewhat surprising.
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