Square's chief financial officer, Sarah Friar, is leaving to become chief executive of neighbourhood social network Nextdoor. Square's stock price fell on the news, but the company had a massive 2017 with its share price tripling and posting $2.2 billion in revenue for the year. Friar guided the firm through its IPO and told Business Insider last year that Square's success boils down to financial discipline and considered expansion. Square is best known for its physical card readers which let small businesses take card payments through a smartphone or tablet.