Keith Rabois, Square’s COO, in an interview for Business Insider:

BI: Have you guys done any advertising? KR: We occasionally test advertising in different channels but the primary driver of the Square option is actually seeing Square in the real word. BI: So no sales force. KR: No sales force, none whatsoever. We have bought an online ad here and there but not in any serious sense.

As a reminder, Square has more than 1 million customers and is processing about $4 billion in payments per year. All of this mainly thanks to word-of-mouth. Crazy!