[eftpotrm]

No, it's not significantly more difficult than the existing case.

You have one permissions set - my permissions on my photos. And another permissions set - my friends' visibility sets on their tagging. The display set is the interaction between these two sets, which any database developer should be trivially able to calculate.

I don't for one minute dispute that the scaling issues for calculating these interactions for 750m users are significant, but the core task definitely isn't.