We have a number of custom audiences created from uploaded hashed emails which are shared with partners. We've noticed that some of them shrink dramatically. For example, one had 4.3 million matched users and after a couple weeks, it's showing 650k. Another went from 5.4 million to 540k. Has anyone else observed this? I'm trying to understand what might be causing this. Could the partner's targeting be affecting the audience? Could it be opt-outs?