http://www.nytimes.com/external/readwri ... 78938.html
Security researchers at Symantec recently uncovered a backdoor trojan whose spread is being dictated by commands hosted in Google Groups, Google's online discussion forums. The backdoor trojan, named Trojan.Grups, appears to the be first ever malware to use an online newsgroup as the "command and control" center for botnet communications. It's certainly the first time that Google Groups specifically has been compromised in this way. This new discovery points to what appears to be the latest trend in what you could call "Web 2.0 malware," that is, nasty computer programs that don't just spread in social networks, but actually use the infrastructure of the social networks themselves to do the spreading.

It goes on to say that you would use social networks as the command and control for botnets because you can't just block off facebook or ban twitter from your internet. You have to rely on the owner of the network to remove the offending user themselves.