The Brazilian digital bank Nu Holdings (NU 1.72%) went public at the end of 2021 with a huge valuation of $41 billion and the backing of Warren Buffett and his company, Berkshire Hathaway, as well as a number of top venture capital firms. Since then, however, the road has been anything but smooth, with the stock down about 50% this year at Friday's prices.

Despite the stock's performance, Nu has continued to put up solid growth and results that show clear progress in what the company is building. Now I think there are 65 million reasons to invest in Nu. Let me explain.

Incredible customer acquisition

Nu got its start by offering a sleek digital experience that made it easier for customers to bank in Brazil -- and at a much lower cost, because Nu offers lower-fee products and services than the large traditional Brazilian banks. As a result, Nu has provided millions of customers with their first-ever credit cards and bank accounts. Now the digital bank is spreading like wildfire in Brazil and starting to expand elsewhere in Latin America, a region that many consider to be one of the next big banking markets, with more than 666 million people.

Warren Buffett.

Image source: Motley Fool.

In its recent second-quarter earnings report, Nu said that it now has more than 65 million customers and banks roughly 36% of the Brazilian adult population. Roughly 63.3 million of Nu's customers are consumers, and about 2 million are small- to medium-sized businesses. That's impressive growth for a company that was founded in 2014 and had only 6 million customers in 2018.

More than 52 million of Nu's customers in the second quarter were defined as monthly active customers -- those who have generated revenue for the company in the prior 30 calendar days. Roughly 55% of these active customers are using Nu as their primary bank.

What's even more impressive is that a lot of this growth is organic, which has given Nu an industry-low customer acquisition cost that management has said in the past is around just $5 per customer. I can't think of any other fintech company I follow doing better than that.

Furthermore, Nu's servicing costs per active customer come to about $0.80. Nu CFO Guilherme Marques do Lago said on the company's recent earnings call that this is about 85% lower than traditional banks, although management expects the company's customer servicing costs to rise to about the $1 level going forward.

Nu is also starting to grow in other parts of Latin America and is now the No. 1 new issuer of credit cards in Mexico (2.7 million cards) and Colombia (more than 313,000).

Customer acquisition is starting to translate

The strong customer acquisition capabilities of Nu are starting to translate into stronger financial results. In the second quarter, the fintech company generated record revenue of $1.2 billion, which is up 230% year over year. The company still lost about $30 million in the quarter, but on an adjusted basis excluding stock-based compensation, Nu generated an adjusted profit of $17 million. That's similar on a year-over-year basis, but Nu also had about $17 million more of stock-based compensation in the quarter than in the second quarter of 2021.

Also, Nu's Brazilian operation generated an accounting profit of $13 million in the first half of the year, and the company's core credit card and personal loan products are also "very profitable," according to management.

Nu's incredible, efficient customer acquisition and the massive market opportunity are likely big reasons Buffett and Berkshire are investing in Nu. Berkshire reportedly invested in Nu when the company was valued at about $30 billion, so investors have the opportunity to buy into this Buffett stock at a cheaper valuation than the Oracle of Omaha himself.