Sometimes praise delivered by word of mouth is the most effective form of advertising. Word of mouth is especially valuable if it comes from a respected voice with a large following.

In late August, Mark Zuckerberg, co-founder and CEO of Facebook, expressed his approval of the iGrill through a comment on his personal Facebook page.

The iGrill is a product designed by iDevice that acts as a technologically advanced meat thermometer and works hand in hand with the iGrill app. Some of the unique features of the iGrill include Apple, Facebook and Bluetooth connectivity, as well as recipes and cooking time remaining. Zuckerberg's praise caused a flood in traffic to the iGrill's website. Two hours after he made the post, the iGrill website crashed due to an overwhelming number of visitors.

Zuckerberg isn't the only celebrity who uses social networking sites to rave about products he loves. Plenty of other high-profile figures are using social media to spread the word about products and services that they enjoy. Here is a look at five other celebrities who are advertising products they love, free of charge, through social networking.

Olympian Rebecca Adlington for BMW
Rebecca Adlington is a name you may recall from the 2012 Summer Olympics in London. In June, as London was preparing for the then-upcoming summer games, the Olympic swimmer was praising her car company and the dealership that assisted her. On June 25, 2012, Adlington tweeted: "A big thank you to BMW + BMW Sytner Nottingham. Everyone is always so helpful and nice there."

This is a perfect example of free advertising launched at an impeccable time. As the Olympics heated up and Adlington's popularity rose, she posted about BMW and potentially influenced over 280,000 active followers on Twitter.

Celebrities Love Clarisonic
Clarisonic is another company that received a hearty boost from unpaid celebrity endorsements. Clarisonic, also known as Pacific Bioscience Laboratories, is a Seattle-based tech startup responsible for Sonicare toothbrushes and the Clarisonic skin care device, among other products. Some of its unpaid celebrity endorsements include Oprah Winfrey, Cameron Diaz, Justin Timberlake and Tyra Banks, all of whom shared their experiences with the brand via various social networking websites. Company sales went up from around $1.7 million in 2005 to over $40 million in 2008, largely due to positive word of mouth from celebrities.

Oprah's Favorite Things
Oprah Winfrey is one of the most influential women of the last two decades. She ruled afternoon television with her long-running eponymous talk show and used her influence and power to promote some of her favorite products and services. Near the end of her show's run, Oprah had a list titled "O's Favorite Things," which promoted the many products that she enjoyed.

One such company that benefited heavily from Oprah's endorsement was Spanx. With very little advertising, the shapewear company went from being relatively unknown to one of the most sought-after shapewear brands available.

The Bottom Line
With social networking playing such a central part of our daily lives, a product endorsement from a celebrity, paid or unpaid, is going to reach a very broad audience. If word of mouth was a valuable source of advertising before the days of Facebook and Twitter, it is an even more important part of business today. A company's reputation can soar across social networking websites like wildfire when it offers great products, provides strong customer service and keeps its clients happy. Alternatively, a company's reputation can plummet with just one bad review.

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