As you probably know, you can set an auto response to the messages your FB page receives.
But FB doesn’t let you do the same thing for your personal profile.
If you have many FB accounts, this is needed, as it may take a lot of time to go through all the messages in all the accounts and respond to them.
This free tool may help.
You can set an auto response, something like “I got your message and I will reply ASAP”, or maybe “Thank you for your message, please call 555-5555 to order” etc.