The internet age is also about freedom of information and access to information in whatever form it takes. Like any period of history, there are forces -- government, corporate, or whatever -- that want to limit the access of ordinary citizens, so that they can be controlled and used. An old example was the printing press. No sooner did Europe get presses, than governments and people in power tried to control them, their publications, and access to channels of dissemination. The results were interesting, to say the least -- and led to things like the US constitution's guarantee of freedom of the press.
Same thing we're seeing here 200 years later. This kind of law is not really about controlling piracy, it's about making sure that corporate and government powers retain control over access and dissemination of information -- whether that's to control the minds and opinions of their populations, or to control the flow of money into the coffers of entertainment conglomerates. The conglomerates don't really "need" the supposedly lost revenues form piracy. They're still making money hand over fist -- they just want ever more.
What we need to protect, ultimately, are the small individual artists, their livelihood, and the eventual timely passing of previous generations' works into the public domain to become the fodder for future art. Corporate interests stop all of that natural cultural flow by being greedy, powerful, and effectively immortal.
I have read several articles on the subject, including this helpful infographic: [link]
The question is less about the specifics of SOPA (there have been several previous attempts at similar legislation) and more about the continued attempts by special interest groups to control the internet. The SOPA legislation will be damaging to Youtube, Twitter, Facebook, Google, Ebay and whatever new services come along in the future. The internet needs to remain a Wild West if it's going to continue to grow. If corporate interests, like Microsoft for instance, take control, everyone's access to information, outside of mainstream media outlets, will be severely compromised.