I agree that Facebook might be scared but let me be the devil's advocate and propose that maybe they are just uninformed.
Having posts rewarded with what might seem real currency can easily lead to people spamming their posts everywhere and Facebook might be trying to avoid that...
I don't see STEEMIT as a direct competitor to Facebook so I don't understand why they would do it deliberately. They can both peacefully coexist. It is probably a misunderstanding and communication with Facebook is the key to solving this.
RE: Let's Talk—Why Is Facebook Treating Steemit URLs as Spam? (UPDATE: It's Deleting My Post In 30 Days Now!)