True, advertising makes up a huge portion of Facebook and twitter's revenue along with data sales. As SteemIt's userbase grows advertising could become an option for them, however. Even if they want to avoid direct advertising they could still do a lot with partnerships, given that they have an active form of currency tied into the platform itself. I doubt that they will ever resort to selling data the way that Facebook does, hopefully, but they can still make a pretty penny off of a few advertising models.