I have some experience on this field. Around two years ago i was a DevOp for the company running Dagbladet, Norways #2 newspaper. One of the things I did was keep an eye on mysterious traffic. I managed to find a huge spam network that set up a proxy service that delivered normal content, but injected "you can win an iPhone!" spam to all users visiting them. Since I was in the position of being able to monitor their…
The reason you found so many domains is that they intentionally take down thier spam sites and reload them under a new domain every few hours. They do this so they can't be taken down by people reporting them as spam. They literally setup the next domain while the current one starts being used so they can do a live swap to the next one without interruptions to thier spam operations. This is typically done in an effort to spread Trojan malware to anybody running computers with out of date operating systems and browsers. Windows getting people off of Internet Explorer has been a huge hit for them as it reduces the number of possible vulnerabilities someone might have when they get sent to one of these Trojan spam sites.