Main question: Is there a character limit on Facebook private/direct messages?
Does this differ when submitting a message on a mobile device as opposed to a regular desktop browser – or on the message page as opposed to the chat window that appears in the corner of the screen?
What happens if a message is too long (i.e. is there any kind of a notification, or does it get split up)?
I've seen some conflicting but limited responses to this question online (though most on Stack Overflow seem to apply to APIs/applications, not regular usage), so just looking for some up-to-date clarification at the tail-end of 2014. I'm NOT asking this question in relation to comments or wall posts.
Best Answer
The limit is 20,000 characters.
This is either something only a Facebook engineer will know or someone with a lot of time on their hands to test every scenario, mobile device and browser out there. It's a safe assumption to say no.
If the message is too long, it will not send, and an error message will appear.