Facebook and Google are data companies. They use have some of the world's best data scientists analysis their user data looking for patterns. On Friday, Facebook announced (link) that they can predict when 2 users are about to change their relationship status to "in a relationship".
3 months before a couple updates their relationship status on Facebook to announced their undying love, there is a steady increase in the number of timeline posts shared amongst the 2 lovebirds. This cross-posting will peak 12 days before the actual change is made. It seems once the relationship status is updated on Facebook, the number of timeline posts between the 2 drops.
"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."