SOPA will indeed have catastrophic effects on a broad spectrum of websites. The main effect will be that many websites will go behind a “walled garden” members-only environment. A substantial portion of those that don’t will simply shut down and disappear. Essentially you will have only government-owned and controlled websites in the USA or private members-only websites.
I’m not sure what effect it will have on advertising, only that it will be very negative due to the high costs and regulatory requirements that every website operator will be forced to comply with.
The number of ISPs will shrink, the number of blogs, forums, and web 2.0 properties will shrink. Social networking sites will be shuttered and the overall use of the web inside the USA will be oppressed.
While I support the stated goal of SOPA, the collateral damage to the web will be catastrophic and I would loathe the passing of that ill-conceived bill.
The activity that the bill targets is already illegal, they don’t need a new law that makes honest ethical practices illegal or prohibited due to regulatory burden. If it passes say goodbye to most of your favorite websites.