I've been thinking, as well as having a general chat room where people can dip in and out, what you need is some kind of trigger for a chat with a specific subject matter that can act as the fulcrum around which those interested can get to know one and other. So perhaps you make a post about a date, time and subject matter (maybe a brief summary of the last discussion) so that everyone can meet in the chat room and you do that maybe once or twice a week.