type of communication probably depends on your type of band and how close you get. If i remember correct Blixa Bargeld dropped out of The Bad Seeds by dropping them a short email, after playing with them some 20 years... as they explaing no bad blood, but the just play together and are not very social... anyways i would suggest face to face. But I have in the past "successfully" dropped out of bands via email, and been rehired couple of years later with no hard feelings. I guess it depends.