My advice (grain of salt) is that you point these events and moments inward to Pair – have Facebook, Twitter post to Pair (e.g., "Post messages I leave on my partner's wall to Pair") or Foursquare check-ins (e.g. "You and your partner were at Giant Tiger together on May 9, 2012". I understand that Pair is for 1-to-1, but since there is no community element, it's hard to stay engaged. If Pair was more like my memories box I think it has legs. You could kind of think of it as an Evernote for relationships.
I also think you should really try to have Pair accessible faster. Having other apps post to it is one way (people are happy to have Facebook open, for example). Another way is to make it as fast as possible for a user to be able to post memories.