That being said, Facebook does let you chat via Jabber. I use it (with Messenger on OSX), and it works, but it's a completely mediocre experience devoid of what makes FB Messenger special.
An example: FB lets you send one-click "likes". It's a great feature; it means "acknowledged". Yet you can't send it with third-party clients, and third-party clients receive it as a URL to an image of a thumbs-up. There's dozens of features like this. How frustrating would it be if Facebook Voice Calls only worked with some people, or someone's client didn't support group messages?