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!