QSL cards first came into existence in the U.S.A., after the end of the First World War. The earliest that have survived are from 1920 - these were basically just hand-written notes on postcards; with the explosion of interest in experimentation and the drive to transmit for further and further distances - initially on medium wave frequencies - exchanges of information and confirmation of contacts by postcard became an important part of the hobby.
The cards below - from the 1921-1925 period - are from operators who later became famous world-wide.