I'd argue that they consciously make it work for them internally, despite lots of pain points solved by other tools better suited for the job.

For example, Zuck says he runs the company via Messenger - but a chat tool without selective muting/@mentions (or equivalent) is bound to be messy at the scale at which he's working. Similarly, Facebook's group chat isn't the best archival tool, nor does it support features like merging posts or transferring them from one group to another.