Facebook has a neat feature called Friendships that lets you see all the interactions you have with a specific person. While it may be obvious how to access it on the desktop, Facebook for iOS doesn't make it quite as easy. The option is certainly there though. Here's how to access it:
That's all there is to it! You are then presented with all the interaction between you two in posts, photos, statuses, and more. It's a neat feature to check out with close friends or family members. I've been surprised at the photos I forgot existed, for better or for worse. It's also an easy way to find a photo you know is floating around somewhere of you and someone else.