So, in other words, this is the price of doing business?
The other option is to ban the use of minority languages. Only major languages are allowed. But, from a human rights perspective, that is an affront to diversity and freedom, and is contributing to the endangered state of many of these languages. But, if you allow minority languages, you run the risk that people are advocating or organising human rights violations in that minority language (slave trading, advocating for genocide, etc), and the platform can't stop it because it can't understand it. So, it is a tradeoff, which threat to human rights do you prevent, because you can't feasibly prevent both.
With this in mind it looks like the occasional slave auction is just an unfortunate consequence of the scale and ambition of the enterprise.
According to Ethnologue, there are 401 languages with over 1 million speakers, and 1370 with over 100,000. Now, can Facebook justify retaining moderation teams with skills in 401 different languages, even 1370 different languages, when most of those languages probably make very little advertising revenue, not enough to pay for the required moderation?
Should Facebook be forced to provide moderation services, at a loss, for lesser used languages? Or, should they ban the use of those languages, where advertising revenue is insufficient to pay for moderation? Or allow them on the platform unmoderated, at the risk that their speakers might get away with saying or doing some horrible things?
I don't think Facebook should be forced to provide moderation services for languages with little or no revenue, and I think allowing them to be used unmoderated is the lesser evil than banning them.
What language were the postings in? The article screenshots one English post, but possibly many of the others were in some local language. Facebook might not employ anyone with language skills in that language. Automated translation software often performs poorly with less common languages, if it is available at all. Even if the postings are all in some major language, they may have been using local slang or jargon, and unless Facebook has staff in or from that country, they may lack the cultural context to understand properly what is going on. Even if someone from that country reports it, it could be very difficult for Facebook's staff to fairly judge the report if they lack the necessary cultural and language skills to interpret it.