Every follower tracker answers its questions with set operations, whether it admits it or not. When we built the historical comparison in Follower Change , the engineering turned out to be the easy part — a few set differences over normalized username lists. The hard part was epistemology: making sure the words in the UI never claimed more than the sets could prove. This post is the data model, and the honesty rules that fall out of it.

Two lists, three questions

A platform data export gives you, for one moment in time, two sets:

F — your followers

G — accounts you follow ("following")

From a single export, exactly one comparison is sound:

G \ F — accounts you follow that don't follow you back. This is a statement about now .

Nothing in a single snapshot describes change.…