I think Facebook is probably shadow banning Steemit content because it's competition. It's obvious that the company wants to limit the growth of any competing social media service. While Facebook does not exactly ban other competing services, as in you can still post Twitter and YouTube links, they typically favor and prioritize their own service. For instance, a video hosted directly on Facebook get significantly more views and shares compared to a video hosted on YouTube.
It would not surprise me at all if Facebook is doing something similar to Steemit to prevent the competition from growing.