It's not obvious? Facebook has enough of your conversations to fingerprint your writing style. Now they can de-anonimize your reddit account and serve you even more personalized ads.
I mean, I don't see a plain facebook connection, but I see googleads and amazon connection and don't know what else obscure things. I doubt it would be hard to sneak something in, that just checks whether this is the same browser where you just switched over from the facebook tab (where the onblur event just got captured). But again, I am not an expert in tracking ads, nor reddit, I just know the web and its various data transmitting technices quite good.