As far as I can tell, the Russian equivalent of the English phrase "to add smb" with regards to social media accounts like Facebook, VKontakte, etc. is "добавлять/добавить в друзья." My question is: why is друзья in the nominative case? Why not в друзей? Is it because друзья isn't being used to mean "friends" in the animate sense of "intimate acquaintances," but instead has taken on a second meaning -- "friends list" -- which would be inanimate and thus not change its form in the accusative case?
You're on the right track, but the meaning is more abstract than "friends list"; it's more like the "state of being a friend" in general. Compare записать в члены общества, вывести в генералы, etc. It seems to be one of the "lesser cases" in Russian (like the partitive-genitive выпить чаю and the locative-prepositional в снегу). I might call it translative-accusative but that's just my own ad hoc term. It's been discussed here: What form is “в гости”?
I actually disagree with @NikolayErshov on this.
While we indeed have in Russian "small cases" for this particular example it's easier. It's actually because it's short for something like "добавить в [категорию] друзья". The same happens when you, say, adding files for some designated folder - when one says "добавить в "Избранное" - he/she actually intend to say "добавить в [папку] "Избранное".