Neobanks, better established across the Atlantic in Europe, are starting to grow up in the U.S. as well.

U.S. challenger bank Chime is growing by 100,000 accounts per month, according to CEO Chris Britt. The San Francisco-based challenger bank just crossed the one-million accounts mark, Britt told Bank Innovation, over a span of less than four years.

The average Chime customer does about 40 transactions per month on its mobile app, Britt said, he doesn’t specify what qualifies as a transaction. So far, it has processed over $4.5 billion in total transaction volume.

“Almost half of these new accounts come from our referral programs or organic traffic,” Britt told Bank Innovation. The program gives rewards to bank users for referring friends to the app, and is all done via mobile, Britt said.

Launched in 2014, Chime, like its peer neobanks Aspiration and N26, Chime was created to cater to the “frustrated traditional bank consumer, bogged down by fees and extra charges,” Britt said. Given that these neobanks/fintechs do not have the larger marketing budget that a traditional bank does, it makes sense that referral program plays an important role in customer acquisitions.

German challenger bank N26, which is set to launch in the U.S. later this year, also owes its growing numbers to referral programs. In fact, N26 U.S. CEO Nicolas Kopp previously told Bank Innovation that most of its users come as a result of its referral program, which involves incentives and gamification tactics to encourage more customers to use it. The bank currently has more than 850,000 users.

Aspiration, another U.S. based neobank, although does not disclose its user numbers, and doesn’t not have a savings account feature. The bank has a total of about $350 million in assets under management.

Of course, these numbers do not compare to the mobile users of the larger banks. Chase has more than 30 million mobile users, Bank of America has 24 million mobile users, and even Citibank, which is well behind its competition in this area, has 10 million users.

Chime does not have a banking license but instead works with Bancorp Bank as its bank partner. Visa issues Chime’s debit card.