super unlikely that FB is using the filenames in any way
First, we don't know that. Second, it doesn't matter even if they are not using it in any way. Backblaze shouldn't be sending this data to FB. And lastly, even if FB isn't using it in any way today, how do we know they won't use in future?
Nobody should be having access to any data that they don't absolutely need. It doesn't matter how mundane the data is, which company the data is going to, etc etc.
More so with all the machine learning code being deployed to gather better information about people to thrust more ads on them, it's possible that even those who work at Facebook may probably not know the extent to which all this data is funneled into various systems and attempts are made to extract monetary value from it.
Like I said, it's definitely a problem regardless.
But this is like saying it that if you misplace your cell phone, it doesn't matter whether you left it in your friend's car or a taxi. Of course it matters whether or not data is being (mis)used.
First, we don't know that. Second, it doesn't matter even if they are not using it in any way. Backblaze shouldn't be sending this data to FB. And lastly, even if FB isn't using it in any way today, how do we know they won't use in future?
Nobody should be having access to any data that they don't absolutely need. It doesn't matter how mundane the data is, which company the data is going to, etc etc.