So far, I think the daily emails are enough of a reminder. Even if you skip a day, it comes back the next automatically. With a paper journal, if you forget one day and it gets buried there is no external process that's reminding you to do it. (Except your then guilt ridden conscience)

Also, I don't think the dynamic of having a friend involved would work to well. Why would I like to be bothered if my friend can't keep his commitment to write a journal. As soon as it becomes a habit and they don't fix it, I'm just going to mark it as spam.