When Matt Fisher blogged last Monday about his sister's death in an auto accident that her insurance company fought to pay out on, he didn't just air his grievances over the acts of insurer Progressive — he placed a spotlight on the power of social media.

Fisher's message spread like wildfire last week after he published the incident. The news gained momentum on Twitter, on TV, and generally across the internet. People were outraged, and with good reason.

"There isn't much Progressive could do about it, other than... not be a bad actor, but for everyone else, the second lesson is to understand the power of the customer," says Nicole Sturgill, a research director in the retail banking and cards practice at CEB TowerGroup.

Fisher was using the tool Tumblr. Akin to Wordpress, it's a platform that lets bloggers self publish. However, on Tumblr, as on Facebook, commenters are required to share the blog post on their own pages.

"Most banks have no idea if their customers even have a Tumblr blog or a Twitter feed. They should absolutely be adding that to their [customer relationship management] applications because customers not only have monetary value to them but they also have PR value," says Sturgill.

That means a bank needs to know if the customer is likely to talk about them, and whether it's good or bad.

Indeed, this event is just the latest in a series that should have banks paying attention, says Frank Eliason, Citi's senior vice president of social media. Another recent example is the Chick-fil-A case in which equal rights activists derided the chain restaurant.

"It is not new to see customers taking control of a brand via social, but the sheer growth is what brands should be thinking about," he says.

Eliason says all of this should cause banks to think about social media outside of Facebook, Twitter and YouTube. He says the first step should be listening — using tools that monitor a bank's brand over the internet.

Other than that, he says, "companies must be prepared to respond properly to address concerns. The customer owns the company message via social media."

Banks have only recently begun to pay closer attention to social media, according to Andrew Somosi, the chief executive of NM Incite, a joint venture of Nielsen and McKinsey & Co.

“It’s early days, but I think there is tremendous appetite that is just starting, really,” he says.

“Everything from fraud development to sales and branch management, we see that there is a very large set of use cases.”

Already BBVA, Citi (C)  and American Express (AXP) are leveraging sentiment analysis tools that allow them to dig out positive and negative messages that are being shared about their businesses across the Internet. 

With more and more under-banked and unbanked, people finding their voice over the web, and social media listening is more important now than ever before, says Jim Van Dyke, president and founder of Javelin Strategy & Research.

“You see social media starting to mature as a way of galvanizing crowds,” he says.

Van Dyke adds that customers that might not have as much of an impact on a bank’s balance sheet can, inevitably, affect a bank’s bottom line by affecting its reputation.

“You can’t just stick to the actuarial tables,” he says.” You have to say: ‘How would it look if this particular issue was blasted out everywhere?’”