As the social media marketing gold rush continues, one of the marketing industry’s top executives — WPP CEO Martin Sorrell — is questioning the medium.

“Invading these [social] media with commercial messages might not be the right thing,” said Sorrell at an event this week hosted by the Financial Times.

Sorrell added that social media sites are “less commercial phenomena, they are more personal phenomena,” more akin to “writing letters to our mothers” as opposed to traditional mediums such as television.

As the Financial Times‘ Tim Bradshaw writes:

This would make Facebook and other social media more of an opportunity for public relations agencies – a smaller part of WPP’s empire – than traditional advertising.

Despite Sorrell’s stance, WPP has gobbled up a number of social media companies, including measurement firm Visible Technologies, social ad network Videoegg and private label online social network company LiveWorld.

At a media event this time last year, Sorrell said the three main areas of focus for WPP are emerging markets, “consumer insights,” referencing the acquisition of TNS, and “new media.”

WPP announces fourth quarter 2009 earnings tomorrow.