SocialMedia, one of the leading ad networks on the Facebook Platform and other social media platforms, has raised an additional $6 million from new investor IDG Ventures and existing investor Charles River Ventures, VentureBeat reports. IDG’s Alex Rosen will be joining the company’s board.

In addition, the company says it closed over $15 million in revenues for 2008, largely driven by its centerpiece “Word of Mouth” advertising products for big brands to engage users on social networks.

Here’s how it works: When a user interacts with an ad in the SocialMedia network, the service is then able to serve a special version of that ad to their friends showing what the person did. For example, this could mean watching a movie trailer or answering a short poll – in a recent Macy’s campaign, users were asked “What kind of Santa are you?”, and their answers were shown in ads shown to their friends.

SocialMedia CEO Seth Goldstein says these campaigns can sell for as high as $20 CPM, compared to the normally low CPMs that most social banner campaigns see. Goldstein explained the benefits advertisers and publishers see with these ad units in an interview with Inside Facebook last year.

SocialMedia launched in mid 2007 as one of the earliest ad networks serving Facebook application developers. The partnership with IDG will help the company expand and deepen its relationships with major advertisers, as it continues to extend its publisher network across the Facebook Platform and, soon, new platforms like Twitter.

(Note: SocialMedia is an advertiser on this blog.)