I went looking today for tools to create an autoresponder for Facebook's private messaging functions. I try to avoid using Facebook's messaging whenever possible, but that doesn't stop someone who I'm connected to there from sending me a private message, which then most often sits unreplied for weeks or months. Having an autoreply that encouraged message senders to email me instead would save me some time and help make sure the contact attempt got through in a timely manner.
The bottom line is that the options are very limited and I may need to build my own if it feels important enough to pursue. In the meantime I thought I'd post my findings here in case there are others looking for the same, or who have new ideas to share.