American Express (AXP 1.16%) and Citigroup (C 0.89%) face countless competitors in the credit card industry. But it turns out Discover Financial Services (DFS 0.33%) may be poised to take them down.

The stunning truth
Many people know American Express and Discover are major players in the credit card world. While Discover has some operations outside of the credit card space -- it has a full-fledged banking arm -- the two are undoubtedly linked and known for their massive dependence on income from the credit cards they issue.

Source: Flickr / S_E_Santana.

Citigroup is less known for its credit card presence, but it too has a massive reliance on credit cards. Across the globe it has a staggering 140 million accounts open, compared to 108 million for American Express. 

In the first quarter alone Citigroup drew in $4.6 billion worth of net interest revenue from its credit cards outstanding, representing more than 40% of its total revenue earned from the interest charged on loans across the bank.

And while Discover may be the smallest of the firms, it turns out it has the biggest reason to be optimistic about its future.

The big win
Each year Satmetrix releases its net promoter score, which calculates the likelihood of a customer to recommend a product or service they've used to someone else. It's based off a survey of more than 23,000 people across the U.S. and their opinions not just on brands and products in financial services, but technology, retail and much more.

It's calculated relatively simply, as it takes the percent of customers who would deemed promoters (who rank the brand as a 9 or 10 on the ten point recommendation scale) minus those who are detractors, with a score of six or below.

And some of the biggest news this year came from previously mentioned credit card industry, as American Express gave up its six year reign as the company with the highest NPS with a score of 45.

And who beat it?

Discover, as it's score rose from 44, narrowly trailing American Express in 2013, to now stand at 52.

So how'd Citigroup do? It certainly wasn't third place. And it wasn't even close, as even despite the 12 point increase versus 2013, it was "the industry laggard with an NPS of 18 points."

Why it matters to investors
The natural question becomes, how does all of this matter to investors? A data scientist at Satmetrix, Brendan Rocks, noted:

The Net Promoter leaders in their respective industries have positioned themselves to outpace their competitors in the areas of increased customer retention and acquisition.

In the case of Discover and American Express, evidence of the strong NPS scores provides one more reason to have confidence in their long-term potential as investments, since having strong customer loyalty is essential for success in the credit card industry.

However for Citigroup, knowing it's the "industry laggard" in a business it really depends on is troubling. Although it's added nearly 7 million new accounts over the last year, it saw its total fall by 2 million during the first three months of the year. And while its accounts were up by 5% over the year, the number of purchase sales rose by less than 2%.

American Express and Discover each offer compelling investment considerations, but with each day Discover provides more and more evidence that it is poised to do great things in the years to come.

On the other hand, investors must keep an eye on this one rarely discussed, but essential part of Citigroup that seems to be in trouble, as its success is critical to the business as a whole.