Meanwhile, 80% of credit unions with assets above $500 million reported annual membership increases and those above that threshold accounted for 82% of all membership gains.

"As much as we hate to admit it, I believe that almost every credit union has drawn their own Bank Transfer Day line in their membership," said Ron Daly, president of DigitalMailer, a Herndon, Va.-based digital communications and electronic marketing firm. "On one side of the line are profitable accounts. On the other are future credit union transfer day candidates."

Daly said while it's true that credit unions' operating costs are lower than banks, there is a point at which they can't be driven down any lower. Marketing efforts, new remote services and technologies to stay competitive can easily add to those expenses, he noted. For instance, mobile banking can be critical to attracting and retain members, but it doesn't have a real impact on the operating costs for a call center.

"In fact, it likely shifts the Bank Transfer Day line for credit unions, moving even more members to the wrong side," Daly said.

Finding the balance between serving members and increasing profitability may mean managing members' expectations while moving them in a direction where they can positively contribute to the bottom line, Daly said. Credit unions may have to do some tweaking to the traditional loyalty plus relationship equals free stuff model to help steer members from what he said is the nonprofitable side of the Bank Transfer Day line to the side that's profitable.

There is evidence that credit unions are having an impact on wooing and retaining members. In 2011, credit unions provided $6.3 billion in direct financial benefits to the nation's 92 million members in the form of lower interest rates, higher dividends, and fewer and lower fees, according to a recent CUNA's "Membership Benefits Report," which compared credit union rates, fees, and dividends to those of banks, both nationally and locally, to determine how much consumers save by using credit unions.

"Despite the uncertainties and a host of new obstacles and burdens, the nation's credit unions are improving member households' financial condition by offering better loan terms, a path to debt reduction and the safety of insured deposits," Colby said.

In the thick of it all, credit unions are growing in size and financial strength.

"At the front of the pack are credit union leaders who have decided to create their own economic recoveries within their communities and fields of membership," Colby said. "Looking back through five economic cycles, in my 35 years of working with credit unions, I can tell you this is how real economic recoveries always begin."