When a user visits the profile page of another user within the web UI, and they do not follow this user, there should be an indication of whether this user is followed by anyone you're following.
This information can already be determined manually by poring through your follows and their followers, but doing it automatically provides a degree of network awareness.
This feature is used on Twitter to provide a reflexive degree of community defense, in that there may be a reason someone is presently excluded from your network. As Marco Rogers puts it:
When twitter says “not followed by anyone you’re following”, take that to heart. It is the only favor this platform does for you.
I'll just add that this should not be provided if the user has chosen to hide their following/followed network, since harassing people for following other people is also a thing that this feature enables. Giving the user the choice to show or hide this leaves it up to them whether they are comfortable making their network public.
That's a great point, Darius.
In the case where a user has not chosen to make their network public, the message could read, "This user has chosen not to make their network public." so that the absence of "Not followed by anyone you're following," is not read as indicating the existence of a shared follow/er.
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I'll just add that this should not be provided if the user has chosen to hide their following/followed network, since harassing people for following other people is also a thing that this feature enables. Giving the user the choice to show or hide this leaves it up to them whether they are comfortable making their network public.