What do you think? Am I all wrong here?
I think you're right. Steemit and Facebook aren't even targeting the same niche, so even if Steemit becomes super popular it likely won't be at the expense of Facebook. The services more likely to suffer are blogging services like Tumblr and Medium.
Thanks for the reply @kapteinb. You have a very good point! As @jetben2016 states:
So FB could be the low-level entry platform that's easy to use by everyone, while Steemit will be more for those who actually post more advanced stuff.
Although if a group of friends join, and they post and upvote each other, then they could post all kinds of stupid shit just to generate money for themselves. I don't know. Let's see how it turns out :)