Social networking sites such as Facebook, MySpace and LinkedIn are fast emerging as some of the most fertile grounds for malicious software, identity thieves and online mischief-makers. The biggest danger from social networking sites is that they are all tripping over themselves to embed powerful features that most subscribers will never use, such as digital image or media files with the ability to download content from third-party Web sites, said Shawn Moyer, chief information-security officer at Agura Digital Security, a Web and network security firm.