Other than doing a daily or weekly message together yourself and sending it out, there is IDEAFit Social Poster. That allows you to post daily or weekly articles and motivational posters to your FB page. I’m not sure if this is what you are looking for, but I hope it gives you some ideas.
Also, you could just create your own daily challenges and share related articles and exercises via e-mail on Client Share.
I have a few clients that like to have a couple of new workout challenges every month. I have them put these things on their calendars with an alarm reminder if possible. I bet there is a trainer service or app that does this as well. I would have to search for it, though. I don’t know of any off the top of my head.