If you disappoint your passengers, "it will be public, and it will live forever," Forrester Research analyst Henry Harteveldt said at the recent SITA IT Summit in Cannes, France. Harteveldt was talking about social media phenomena such as Facebook, YouTube, Flickr and Twitter, which allow travelers to broadcast their experiences immediately to hundreds, thousands, even millions of people.

The reality of social media is that airlines must treat their customers as though the entire world is watching. A good chunk of it may be doing just that. Hartelveldt's presentation, titled "Keeping Mr. 22D Happy: What changing passenger behaviors and attitudes mean for air transport industry IT," included two photos posted by Flickr users. One depicted the airport in Baden, Germany, where, the poster said, "Ryanair gave me a birthday present on Sunday of an extra 5 hours and 40 minutes in Germany. The bad news is that I spent them at Baden airport!" The other memorialized in less-than-appetizing detail a breakfast served on a BMI transatlantic flight.

An emphatic demonstration of Harteveldt's warning came a few days after he spoke in Cannes, when Canadian folksinger Dave Carroll posted a video on YouTube that details a battle with United Airlines in music and lyrics. Carroll's song describes his experience in March 2008, when he and his band traveled from Nova Scotia to Nebraska with a connection at Chicago O'Hare. While sitting on a plane in Chicago, he heard a passenger exclaim, "My God, they're throwing guitars out there!" He immediately alerted three employees that baggage handlers were mishandling expensive equipment, but none took any action. Sure enough, Carroll's guitar was severely damaged.

For more than a year, Carroll tried to get United either to replace the guitar, pay for the repair or provide travel vouchers as compensation. Nothing happened until he posted his video, "United Breaks Guitars," on the evening of July 6. On July 7, United took action, and it, too, used social media -- in this case, Twitter -- to say, "This has struck a chord w/ us and we've contacted him directly to make it right." In another "tweet" on July 8, United said Carroll's video "is excellent and that is why we would like to use it for training purposes so everyone receives better service from us." But by July 10, the video had been viewed more than 1.38 million times, and more than 14,000 viewers went to the trouble of rating it (it got five stars).

A Google search for the terms "Dave Carroll guitar United" returned 65,700 links. The video made the rounds on Facebook, too. Travel writer Peter Greenberg posted a story about it for his 1,652 friends.

Carroll was invited to appear on CBS News' morning show, where he said that were it not for the video, he was sure United would not have gotten in touch with him. "They told me I wouldn't ever hear from them again."

The moral of the story: Passengers are now armed with the ability to air their complaints instantaneously and globally. "Public relations no longer controls the message," Harteveldt said in his presentation. The message is viral, and it lives in cyberspace for a long, long time.

The flip side: Social media can be used as a vehicle to reach out to customers, Harteveldt said. JetBlue and United, for example, alert their Twitter followers to short-term offerings.

Even more intriguing is the ability to use Twitter to nip customer service issues in the bud before they become public relations nightmares. When a customer tweeted, "JetBlue, I need a wheelchair," she got an instant response from the carrier's Twitter account. "Twitter is becoming the customer service feedback loop," Harteveldt said.