This question has been asked before, but it doesn't bring up several important points. As things currently stand, deleting a Facebook message only removes it from your inbox but the recipient still has access to it (unless they delete it as well). If the recipient doesn't delete your message, it will stay on Facebook's servers indefinitely. The best argument for not forcing Facebook to delete messages from both sides is that the recipient may still want to have access to them later. There are also potential criminal issues to consider. If the message is in anyway connected to illegal activity, authorities may want to have access to it in the future. However, these are some points to consider:
1) All computer systems are susceptible to hacking. There have already been several data breaches related to Facebook messages. If Facebook can't keep our data secure, they're obligated to remove it at some point.
2) Never in the course of human history has everything you said been saved and recorded forever. At some point, it's reasonable to want your data to no longer be accessible.
As far as illegal activity is concerned, there are other ways to mitigate crime. And with regards to the rights of the recipient, they can save and store the messages for posterity's sake. However, that doesn't mean messages have to be on Facebook's servers forever.
So here's my question: Is anyone anywhere trying to litigate this? At the very least, can Facebook be required to create an easy mechanism for deletion? For example, allowing for automatic deletion requests to be sent to both parties? By the way, Mark Zuckerburg already has this feature. https://www.theverge.com/2018/4/6/17203114/facebook-mark-zuckerberg-messages-deleted-messenger-inbox