A little OT but reminds me of the design implications of Graph Search: the Friendship pages which were introduced in 2010 [0]
From what I remember, these were introduced but after awhile, seemed to be a feature that fell into obscurity. As in, you pretty much had to manually enter the URL. The feature still exists and it's highly welcome, IMO, when FB does the occasional "You and John Doe have been friends for 8 years. See your memories" type reminders.
But to manually see your friendship with a friend, at least on the Desktop version, you have to click the "..." menubox, in which other lesser used features (such as "Poke") are buried. And it's possible to view the mutual content between any two users by filling out this URL manually: facebook.com/friendship/friend1/friend2
But I'm guessing the latter functionality was never given prominent exposure due to how easily it could be used for stalking a current or ex-love interest? Sure, you could scour your targets' walls individually and make a note of all public photos/messages in which they mutually appear. But the Friendship view streamlined that process as much as the original News Feed streamlined old-fashioned profile browsing.
I'm having a hard time imagining that it was a feature that would be popular for innocent reasons -- I'm already inundated with news items about friends' activity on my newsfeed, why would I want to view a highly scoped newsfeed about two people who are not-me for innocent reasons, even if I were extremely bored and Facebook were the only site on the Internet?
[0] http://www.adweek.com/socialtimes/friendship-pages-mutual-co...