Once upon a time, big financial institutions feared competition from just-as-big companies gunning for their customers. But threats from giants like Walmart and Google haven't materialized. In fact, the biggest threat is now coming from garages and strip-mall offices filled with 20-somethings developing pinpoint mobile apps that customers are using to chip away at the traditional financial services model.

In 2007, in the face of pressure from regulators and financial institutions, Walmart abandoned plans to form its own bank. But ever since then, the top banks have kept wary eyes on large enterprises everywhere that might threaten their hold on their customers. In fact, Walmart, while ditching its plans to take deposits, has developed several payments-based products that are nearly enough banking to satisfy their customers.

But while banks continue to keep an eye on the giants, they may be losing sight of an even more powerful threat, something that could perhaps be called "social finance," coming from the world of mobility. It seems like every week there's an announcement around some new smartphone app that can handle personal financial management, pay your bills, send money to a friend or relative, watch over your credit, or even fund your small business, all without going through the traditional banking players.

Of course, there has to be a bank somewhere in the background, processing the transactions, forwarding the payments, etc. But I would argue that this battlefront is the customer interaction. For years, banks have understood that part of the value of the branch and ATM was in their brand strength. And this is the very battle being fought. Only this time, it's not against a Goliath, but against hundreds of Davids competing individually, and, like David's sling, using very focused tools to defeat the giant.

Whether the customer is reviewing this month's budget using Mint, checking a questionable credit card charge using BillGuard, or keeping abreast of their crowd-funded project on Kickstarter, banks stand to lose the all-important face time with their customers.

One key to making sure that the bank retains its customers' attention is to look at what they're accomplishing with the apps and offer the same experience. It's clear from these offerings and others that customers are choosing to interact with purpose-built applications that are in line with their daily activities.

While banks work on their next-generation online banking and mobile applications, they need to emulate that aspect of these apps and at the same time leverage the trust that the customer has with the institution. By paying attention to the nimble and tech-savvy Davids, as well as the Goliaths, banks will retain and grow their customers' loyalty.