There’s been a lot of interest among users, the media, and potential investors about social networks like MySpace, Facebook, and others recently. But there seems to be one class of people losing interest very quickly in social networks: advertisers.There was a time when Mark Seremet considered MySpace one of the best things to happen to his business. Seremet, then-CEO of customized clothing company Spreadshirt, saw sales jump sixfold in late 2005 and early 2006 after he ran ads on the popular social networks