The technique I have seen is people post a message on an old photo, wait two minutes for the message to be sent via email, and then delete the message with the photo if they don't want it publically available.
A number have begun to find each other on Facebook where the message system is more sophisticated. If I'm replying here I usually put a comment on the last picture of the recipient - not ideal but better than nothing....
Either put a message on one of their pictures, or check their about me and see if they have an e-mail address, I know that I've written mine there and others might too! :)
I usually answer the question in comments of the picture in question, then I go to their picture from the same day, make a comment then reference their question and answer it again.
Good idea, Andy! It would answer the problem of not turning 365 into another Facebook but would allow for us to message directly and allow for replies easier. There has to be a better way! =D
My email address is on my profile so that someone can email me if they'd like.