Maybe this is something that is visible on a large scale but appears more or less random when zoomed in to a particular couple.
I would guess you are correct: that the signal is pretty weak per individual couple (to the point of not being usable) but shows up with this nice tight graph when you aggregate data.
I was thinking this. The graph looks nice until you look at the numbers on the left.
I guess it is a change but that is still just posting 1-2 times on the wall per day.
Given that the number of posts dips over time, that's a difference of approx 8-10 posts in that 85 day period after the relationship begins. Seems pretty negligible.