Totally serious btw. Right now FB plays a pivotal role in how people declare their relationship status. They already have tons of photos, they already have the users, they already have an ad model that is way more profitable than OKCupid or PoF. They also likely have the data and ability to determine when someone might be about to break up and how compatible people might be. And any added fields for self-reported compatibility data would be gravy for their targeting.
It also seems they have an advantage over most online dating sites in that they don't have the problem of it being more profitable for users to not find their true love. If a couple stays together, they get married, have kids, etc. which likely leads to further engagement with FB.