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

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.