Is the scale there though? I know some people use AI like a companion or mentor, but how common is that really?

What makes you think they’d restrict this to logged in accounts? Not so long ago, Meta got caught building “ghost” social media profiles for people who didn’t have accounts on their platforms. They essentially constructed a “missing puzzle piece” based on people this person knew who did have profiles in publicly available information. Building a psychosocial profile on a user who has not set up an account yet using AI would be trivial.