When Gatorade decided to run a series of webisodes featuring a top athlete and buy online ads around it, it turned not to its media or digital agencies. Instead, it tapped Omnicom PR giant Fleishman-Hillard for the work.

It might seem unusual, but it's a growing trend in public relations, where the core competency -- storytelling -- is increasingly cutting across not just earned but paid media. Consider that Fleishman-Hillard last year bought about $100 million in paid media across nearly 25 clients.

"Paid channels are another way to do our storytelling," said Fleishman-Hillard CEO Dave Senay. "We never do paid media on a stand-alone basis. It's just one component of an increasingly integrated spectrum."

PR firms like Fleishman have dabbled in creative advertising and buying before -- particularly in the D.C. market for public affairs. But now PR-agency media buys are moving beyond the Beltway.

Why now? Experts say it's because earned-media efforts, especially social-media work, benefit from being supplemented with paid elements that drive consumers back to the original content.

The budgets that PR shops work with pale in comparison to major media shops, where a single client budget can be north of $1 billion. But PR shops are growing their media operations fast. Fleishman's $100 million in media billings in 2011 is a big spike from the firm's $7 million in ad billings in 2010. Nearly half of the firm's buys were online, and the rest was largely print, with work handled out of New York Chicago, Hong Kong and London.

Publicis Groupe 's MSL Group is also ramping up its capabilities in the space to support the $55 million in media buys it placed last year globally, according to MSLGroup President of Americas Jim Tsokanos. One of the most significant campaigns is for client Underwriters Laboratories, a product-safety-certification company.

Underwriters hired MSL five years ago to support global consumer marketing efforts targeting new moms. At the time, said CMO Sara Greenstein, Underwriters held a review considering various approaches, such as bringing on specialty agencies for each piece of business and "a more traditional ad route." But the company ultimately integrated its combined PR, advertising and media account within MSL.

"It's working for us," said Ms. Greenstein. "What matters less to me is that it's a PR agency. I [care about] finding the message that grabs the audience you're trying to grab and makes an impression. When you think about who historically has been the best communicators and the best at finding messages, that 's PR."

Edelman is also gaining share in the paid space, and recently expressed its interest in acquisitions that could boost its capabilities in digital-media buying and search.

"We have a good sense of how to do this on a daily basis, vs. weekly or monthly," said Alan VanderMolen, president-CEO of Edelman's Global Practices. "Our guys are used to optimizing content."

New buying opportunities mean PR shops will be seeking new types of talent, said Weber Shandwick CEO Harris Diamond. He said his firm's ad buys have nearly quadrupled over the past two years, in large part from digital efforts, as well as social buying in the crisis category. As a result, the firm is looking at hiring two media planners from the ad- and media-agency worlds. "We're creating more content on behalf of clients," Mr. Harris said. "Traditionally, we'd place that for free by earning it, but now clients are asking us to place it [through buying] and we're coming up with the ideas to place it."