What it used to be called:
E-mail rushed in, followed by texting, social network messaging, etc.
Snail mail is not only slower, but also much rarer. In the United States, for example, about 175 billion letters are sent each year, compared with many trillions of e-mails (and a heaping portion of spam).
An early use:
"With the proliferation of personal computers, electronic mail is becoming more viable. As does 'snail mail,' information utilities offer users a unique address." Infoworld, Mar. 29, 1983