This data became public by way of an email snafu - Simple CEO Josh Reich accidentally emailed a draft of a document to a reporter with the same name as "the newest member of [Simple's] finance team" ahead of a board presentation.
The document explained that "customer acquisition is slower than expected" and "deposits per customer are growing slower than expected."
Simple has responded publicly with this blog post titled "How We Measure." In it, Reich explains that traditional banking outlets count someone as an active user if he or she makes "one financial transaction per month," but Simple has found that "[its] customers benefit from Simple most when they use it as their primary bank account[...] Because of this, [Simple sets] a higher bar to measure activity. Simple's active definition takes into account both swipe frequency and deposits[...] Our active customers are already swiping their Simple Visa® Cards multiple times a day, while the average bank customer swipes a few times per week."
The document cites a 29.2% active rate among consumers, so reverse-engineering this puts the total number of customers, active and non-active alike, around 114,339.
Full disclosure here - I'm one of those active Simple customers, and I love the service. The company isn't a proper "bank" in and of itself, but my money sits in an FDIC-insured account, I have no-fee access to thousands of ATMs, I can do free checking by mail, and the in-app savings mechanism is intuitive and awesome. I haven't set foot in a physical bank since switching over.
The company recently merged with BBVA, the second-largest bank in Spain, in a $117 million deal.