On the Internet, there are no secrets. When you are about to change your relationship status, Facebook knows – even before you make the change.
If this seems just a little creepy, consider that you give Facebook a pile of data. Here is how Facebook describes the advance notice you give them:
We studied the group of people who changed their status from “Single” to “In a relationship” and also stated an anniversary date as the start of their relationship. 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
So don’t worry, Facebook hasn’t installed cameras in your bedroom. But what they can see, so can your friends. What and how frequently you post on someone’s timeline might give away that secret flame you’ve been burning for someone even before you change your Facebook relationship status.