Facebook knows exactly how long you're likely to be in a relationship, and it knows where you're most likely to find love as well. But Facebook also knows exactly when you're going to start one in the first place.
This chart from Facebook researcher Carlos Diuk explains how.
In the 100 or so days before you're likely to start a relationship with someone, the number of interactions between users is expected to rise consistently. (This is probably when you're feeling things out or planning how to see the other person more.) Then, right before the relationship begins, there's a free-fall in the number of timeline posts (hanging out in person instead of Facebook, eh?). After the relationship is established, the freefall is followed by a steady decline in the number of wall posts — presumably because lovers are spending more time in person with each other.
And this chart shows how many positive emotions like "love," "nice," "happy," etc. show up in a persons' interactions compared to negative ones like "hate" before and after the formation of a new relationship:
Seems about right.
The sample was taken between April 2010 and October 2013 and only includes those users whose status were listed as "single" for 100 days before and "in a relationship" 100 days after actually mutually confirming a relationship, so the results are likely to be a little biased. Still, it's interesting to see how much control and knowledge Facebook possesses over its users, including a whole bunch of data on just how and when they fall in love.
George Lucas made a movie about something similar.