For me, it's basically network effects - FB is already set up, I can find people quickly (I don't have alternative contact details for some people), I know the recipients active use FB so will see it, and it's easier to send long messages on PC rather than a mobile device.

I also tend to need to message people at odd hours, and a Facebook notification feels less intrusive/urgent than, e.g., an SMS.

Because that's the only reliable way I can message my friends. I'm not sure what else would work, people change/lose numbers all the time. Email is the worst. Not really much solutions out there if you want to keep in touch with your friends.

I personally mostly use messenger.com, rarely use the news feed anymore.

Because everybody else is using FB for messaging? Network effect is a real thing.