Lest marketers underestimate the significance of this strategic shift, it’s worth pointing out that P&G invested more than $8.5 billion on advertising last year, most of it in traditional media. The company not only advertised its products on soap operas, it produced the shows themselves. P&G’s stable includes some of the world’s best known brands, such as Cover Girl, Crest, Duracell, Gillette, Olay, Pampers and Tide, to name a few.

It was only a year ago that P&G started taking social media marketing seriously, opening a Silicon Valley outpost to become immersed in it. At the time, David Hornik of August Capital wrote on VentureBlog, “P&G’s explicit goal for 2010 is to assure that each of its brands has a meaningful presence on Facebook, and they are willing to pay dearly for that. … they certainly view Facebook as a must-have for digital advertising and brand building.”

P&G’s social media pronouncement is likely to re-shape the entire marketing world’s view of social media as a brand marketing medium and, as a result, have a domino effect on marketers large and small. While some early business adopters have been using social media for quite some time, it is a form of brand marketing still in its infancy. That means rules of what to do and when to do it are in flux and measurement criteria are fuzzy at best.

Brand marketers must understand that what they do and say via social media needs to complement rather than conflict with the brand’s image. Social media demands a level of intense, intimate interaction with consumers that makes it all the more challenging to maintain brand integrity.

One thing seems clear: While integrating social media into a brand marketing campaign is becoming essential, it demands a different way of thinking about the lead qualification process. Brand marketers have been accustomed to a fairly straightforward lead generation and qualification structure. Typically, a single consumer learns about a product via traditional media, samples it, and if she likes it, she becomes a regular buyer.

Social media, however, adds a new dimension to the process. For one thing, the consumer enters into a dialog with the brand marketer via social media, creating a responsibility on the part of the brand marketer to respond in a timely, meaningful way. For another, the consumer wields a new authority: She can use social media to influence a much larger circle of friends and acquaintances. Both positive and negative experiences with a brand can be widely and quickly dispersed and “go viral.” That could obviously be a double-edged sword.

When it comes to measuring the ROI of social media, many marketers still struggle, but others are figuring out how to evaluate its impact. Here’s what some marketers who use social media have to say about ROI.

“The ROI is a bit harder to measure, since there is no way to trace whether the buzz we build online translates into sales,” says Cynthia Shannon, a publicist for Berrett-Koehler Publishers in San Francisco, California. “However, we use tools like bit.ly to track click through, and every month I track the number of new followers and use HubSpot Graders to see where we stand in comparison to the rest of the social mediasphere.”

“On a daily basis we track the number of fans and followers from Facebook, Twitter and LinkedIn,” says Lin Parkin, public relations manager for Voices.com, a website that connects businesses with professional voice talent. For example, in just three months our social media efforts achieved over 19,000 Facebook referrals. During that time we had over 778.000 unique visitors to our website and over 4,000 job postings worth more than $1 million.”

Barry Silverstein has been a frequent brandchannel contributor since 2007. He has thirty years of advertising and marketing experience and is currently a freelance writer and marketing consultant. He founded and ran his own direct marketing agency and held executive positions with Epsilon, a leading database marketing firm and Arnold, a major ad agency. Silverstein is the author of three marketing books, including the McGraw-Hill book, The Breakaway Brand, which he co-authored with Arnold CEO Fran Kelly.