First of all, of course truth is a defense, but as pointed out above, perspectives differ, and ultimately a jury would decide.

By virtue of its advertising, an airline would be a "public figure," suffering only if the defamation were malicious, reckless, or willfully disregarding the truth. Even then, however, a service industry business is unlikely to sue a customer, for obvious public relations reasons. They might, however, "fire" the passenger, as US did to a columnist, and confiscate his frequent flyer miles.

On the other hand, private individuals have a much lower bar of negligence to hurdle. That is not to say that such lawsuits are likely to occur willy-nilly, because of the cost of doing so. But when vindictive spiteful posts occur, you never know which airline employee will have a relative or high school classmate who is a lawyer willing to work pro bono.

I am not a plaintiffs' attorney, and don't advocate frivolous lawsuits. Ultimately civil behavior must occur through good manners, respect for others, personal morality, and a well-developed conscience rather than a fear of monetary damages. Unfortunately, there's always a few who lie at the extremes of the bell curve.