As Dawn indicated below, I don't believe there is a definitive answer as that is such a personal question - each member will have a personal preference that you want to be aware of.  My personal bias is that members deserve a personal touch even if it is a message as a result of a phone call....that's subtle marketing.   Having said that, if the communication is important I would also follow up via email or mail if someone does not have email. (Some very elderly folks today still don't use email).