I'm available through most usual channels, and have my particular rhythms for each.
Twitter and facebook are checked several times a week, email and phone messages at least once a weekday. Sometimes it's more frequent, but this is a minimum for reference. So: email and phone calls get a response by the next business day, and facebook and twitter less often.
I have many things in my non-online world that I value highly, and this is my way of keeping things relatively smooth.