Facebook published a Valentine's Day post about how two users interact with each other before and after they make it Facebook official.
During the 100 days before the relationship starts, we observe a slow but steady increase in the number of timeline posts shared between the future couple. When the relationship starts ("day 0"), posts begin to decrease. We observe a peak of 1.67 posts per day 12 days before the relationship begins, and a lowest point of 1.53 posts per day 85 days into the relationship. Presumably, couples decide to spend more time together, courtship is off, and online interactions give way to more interactions in the physical world.
Facebook's data scientists also noted an increase in positive sentiment after the start of a relationship:
Not the most stunning of data visualizations, but a very cool way to interpret part of the huge amount of data at Facebook's disposal. See the blog post here.