Since the advent of public relations, there has been an easy way to explain the profession and differentiate it from marketing and advertising. In fact, I used this method many years ago when I first described what PR to my daughter. My explanation went something like this:

“Advertisers and marketers pay to get their messages to their audiences. In PR, we don’t pay. We earn coverage of our stories.”

The line between paid and earned media was, for a long time, inviolate. There was never any doubt when you crossed the line: If you paid for coverage, you had breached the boundaries of ethical behavior.

Today, a lot of practitioners adhere to this definition. The reality, though, is that the marketplace for information has become far more complex. It is not only acceptable to pay for media in the PR world, it is, in many cases, absolutely necessary.

There is still a line. It’s just no longer as sharp and clearly defined as it used to be.

The role of public relations, expressed at its simplest, is to tell an organization’s story. By offering a story so compelling that others would want to tell it, the story gained credibility the organization couldn’t get from buying space or spending money on marketing efforts. If journalists wrote about it (or broadcast it), the story would be enhanced by the aura of third-party objectivity.

The system worked because journalists held a monopoly on the news distributed by the outlets that employed them. That’s no longer the case, with a splintered media environment offering an ever-expanding number of choices. While many still rely on newspapers and television for their news, others look to sources like Facebook and Twitter, among many others.

In this environment, pitching stories to the press remains important but doesn’t by itself achieve PR objectives. Consider the year-old story of the New Zealand Seafood Industry Council, which took a hit in a New York Times story accusing fishereies of depleting the hoki, the main ingredient in McDonald’s Filet-O-Fish sandwich. The story didn’t acknowledge that the practices had already been addressed and the hoki population was growing again.

To address the inaccuracy, the PR agency for the Council used Google pay-per-click advertising, buying keywords including hoki new york times and new zealand hoki. When a search for these words produced ads that appeared in the paid search sections of the Google search engine results page, a user could click through to a microsite that presented the advocacy group’s point of view. It was pure PR—there was no effort to sell a product, just to tell the organization’s side of the story. Yet driving traffic to the site required search engine optimization efforts, including the use of paid search.

Yes, it’s a step over the old line. But that line has grown blurry.

Since the hoki story, the use of paid search, incentives to bloggers and other techniques have become more and more common practices employed by PR practitioners. These practices are advocated by people like David Jones, who runs Hill & Knowlton Canada’s social media practice, and Edelman’s David Armano.

Other methods have emerged for helping people learn your organization’s story, such as Facebook ads (which can target Facebook users with information about anything from a corporate social responsibility initiative to a product launch) and promoted tweets, which Ford recently used to draw attention to its use of social media. According to a Brandweek article,

Ford contacted social media consultant Mack Collier to ask permission to use his tweet directing readers to a complimentary blog post he wrote about Ford. Collier wrote about a video response Ford CMO Jim Farley (pictured) gave to a question he submitted asking if the carmaker had seen tangible cost reductions in social media.

The Promoted Tweet read, “Ford CMO Jim Farley says social media has led to ‘massive cost savings’ for the automaker,” with a shortened URL link to Collier’s blog post on the subject.

And yet a recent post by Stephanie Schwab (a guest post on Jason Falls’ Social Media Explorer blog) suggests that PR stands to lose its edge as the natural home for social media because of a stubborn insistence that PR deals only in earned media.

Public relations needs to stay focused on telling stories and building relationships between organizations and its core constituents and leave sales to advertising and marketing. But drawing attention to those stories will increasingly require the use of paid media. Knowing when you’ve crossed the line will no longer be as simple as knowing that you haven’t spent one thin dime, but rather knowing that what you spent it on supports PR objectives. It may be hard to explain in a succinct rule or guideline, but, like U.S. Supreme Court Justice Potter Stewart famously noted about hard-core pornography being hard to define, “I know it when I see it.”

This is the new public relations gut-check. We’re still about earning rather than paying for attention, but sometimes that will mean making sure we earn attention by having links to our content appear in the right place—at a cost. Ensuring we don’t cross the line will be a matter of stepping back, looking at the purchase, and knowing when we see it that it has gone beyond PR into the realm of advertising.

It’s one more example of how the fragmented, complex world of digital media has complicated the rules of public relations.