It's not clear to me that Facebook is acting unethically. They provide a communications platform, and communications platforms are always used unethically.

This is true of the telephone. Of the newspaper. Of text messages. Of telegram services. Of video chat. Of magazines. Of public meeting areas like park benches.

Every single one of these platforms have been used for human trafficking and other illegal behavior. It's ubiquitous, always has been, and always will be because the communications on a system are reflective of the humans using them.

The subjective question is whether Facebook puts enough effort into policing their platforms, and it's the exact same question posed (for example) to a city regarding the policing of solicitation in public parks. There will always be people unhappy with the level of enforcement in a common platform. It's reasonable to ask for more enforcement, but I cannot see a compelling argument that Facebook's communications platform is somehow less ethical than any other.