Watch out Bill Gates...

Rise in Social Networks in Underground Economy
It should not come as a surprise that verified, "followed" or "friended" social networking accounts are being bought and sold in the underground, similar to the way that e-mail lists have been for spamming and other campaigns. In fact one ad snippet shows that social networking accounts are more valuable in the underground than email accounts: Facebook ($20 per 100) and Twitter ($19 per 100) versus Yahoo or Hotmail ($15 per 1000) accounts.
I've run into a few of these in the underground marketplace ads, including one today specifically calling out that Twitter accounts are "Good for Spreading Bots."
In light of Bill Gates' adoption of Twitter yesterday, I thought it would be fitting to put out a word of warning: we've already seen Koobface and other malware on Facebook, and we've seen the Twitter/StalkDaily worm. There are many more examples of the social networking medium being used and abused. It is safe to say that enough user adoption of social networking has occurred that the bad guys have likewise adapted and are here to stay on this medium...
Facebook has set 5000 as the maximum number of friends per account, and these maxed out accounts are the among the most valuable accounts being traded in the underground.