Put up a flyer somewhere/put it in mailboxes telling people to call you if they have memories of people who no longer exist. That way you can figure out if this is something that's happened to others as well or if the only disappearances are related to people you are close with.

If it's the first then hopefully you'll have a handful of people out there to work with to solve all of this. If it's the latter then at least you'll know that it's about you. You are the target here, for whatever reason. In that case it's pretty simple: cut ties with people, and they won't be forgotten by others, only you.

I bet there are other people out there experiencing the same thing, though. After all, young children have had "imaginary friends" since forever, right? It's a concept so common that nobody bats an eye. But what if those friends aren't imaginary? Just forgotten, like yours?