Either the wording will be too specific and not mean much, or it will be too broad and harm the innocent.. in a BAD way. Say, a contractor writing to a client "hey, I'm done now, could you wire me the last 50%?"...
They've got no business using Facebook to conduct business in the first place. Besides, their chat and message tools are terrible. If Facebook disallows it in their TOS and it's widely known, then don't do it, especially for business.
Businesses doing business on Facebook is exactly more of the kind of thing Facebook wants, don't kid yourself. They won't pursue a lot of this stuff directly themselves, but they're happy for anything that drives revenue without pissing off users. See: Asana.