Looking at the graph, it seems like we're talking about ~16.5 posts in the ten days before a relationship versions versus ~15.7 in the ten days after. In the context of an individual, Facebook might be able to combine this with a bunch of other data to make guesses about people entering relationships, but a quick look at the graph makes it look like they could do it from just this one piece of data.

FWIW, I think people are often too quick to dismiss graphs as misleading due to the y-scale and I do think the graph is interesting. That said, I think a lot depends on the context, and being directly below "When You [emphasis added] Fall in Love, This Is What Facebook Sees" makes this seem misleading.