No, they don't need to snoop on you. Just fire a single pixel saying this is a user that they would like to find more of. FB does the rest based on data in FB, not on Backblaze. The mistake was them just adding it to the online dashboard because that's the easy way to automatically include paying users.

> "easily identify companies that does this"

No. And this can happen for a lot of reasons as explained so even harder to check for.