How are added and removed VKontakte friends determined?

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There is such a service as 220vk.com (not advertising) which somehow determines when the user has deleted \ whom he added as a friend --> prntscr.com/attyd6
So I sit and think, how is this implemented?
My assumption is that they take an array of the user's friends, then store it somewhere, at the next check they request a new array and compare it with the saved one. But where to store this array then...