I wonder why I have not heard from them again? Why didn't I get a message like " We miss you, please come back, here's a special offer"
The book clubs and the music clubs do this all the time. It would be even easier with email as long as you obey the spam rules.
I'll bet if you do this today with your membership site, you'll pick up some additional revenue for little or no cost.
In addition, that email should have links to other sites that you are an affilate for in case they really don't want your site, but might be interested in another.