Ever wondered how spammers can be profitable? According to this BBC article, researchers have calculated that those sending unsolicited e-mails can make a profit by attracting just one sale from every 12.5 million spam e-mails sent. Even though their conversion rates are well under 0.00001%, big spam networks generate more than $3.5 million USD in annual revenue. All it takes is thousands of hijacked machines sending hundreds of millions of spam messages a day. More details in the original research paper (PDF) by Chris Kanich et al. Crazy!
— Dries Buytaert