@evan This is a good one and a common mapping exercise. Many services don't think this through and / or opt for an odd and challenging option of Bob's followers getting the visibility to it, which pretty much breaks the “followers only” intent of the original poster.

I’ve walk through this in a couple workshops around researching in social media as following the shadows of social media. You can't see a person, but you can see their shadows and essence of their moves and existence.

@vanderwal actually, most social networks default to having Bob's reply visible to Alice's followers. That is how followers-only posts work on X, Instagram, and Facebook.