Citibank and Wells Fargo become the next two financial giants to implement Apple Pay into their mobile banking systems after MasterCard, Visa, and American Express, reports Inferse.

Wells Fargo customers received an email Friday informing them that Apple Pay will now be available for iPhone 6. Citibank customers, however, will only be able to use Apple's mobile payment system if they reside in mainstream U.S. territories, including Hawaii.

Aside from the aforementioned financial establishments, several retailers and service outlets have also signed up for Apple Pay, including McDonald's, Petco, Uber, Staples, Walgreen's, Starbucks, and Whole Foods.

The secret to Apple Pay's early success, writes Eric Auchard of Reuters, is that "Apple Pay preserves many existing relationships while inserting a new dominant player - itself - as kingmaker. Its potential audience? The 800 million Apple users who have already connected credit and debit cards to iTunes accounts."

Nathalie Reinelt of banking consultancy firm Aite Group adds, "Apple's approach appears to be collaborative with traditional payment networks, instead of competitive, and it's much less disruptive."

"Maybe it's also because Apple Pay is easy to use. Services are only really effective if people use them. Apple made it easy for users to set up and use the payment service, making it likely that they'll actually use it," writes Dianne Depra of Tech Times.

"Apple is connected to 800 million existing users and that number could grow. Even if just half of those use the service, Apple Pay would still have a significant subscriber base, and that's what everyone wants a piece of, really."

Consumers with Apple Pay can make purchases by holding up their enabled devices to compatible readers in approved stores and establishments. It was first unveiled on September 9 this year when Visa, MasterCard, and American Express officially declared support for the mobile payment system, and was launched Friday on the new iPhone 6.