SOPA was proposed as a way to censor access to sites like The Pirate Bay.
If SOPA was to pass, copyright holders could complain and have websites shutdown. Additionally, payment processors and search engines will have to block those sites, as well.
Outside of Google and Facebook - AOL, eBay, LinkedIn, Mozila, Yahoo, Twitter and Zynga filed the formal complaint with key Senate and House lawmakers.
Reads the complaint (via CNN):
We support the bills' stated goals. Unfortunately, the bills as drafted would expose law-abiding U.S. Internet and technology companies to new uncertain liabilities [and] mandates that would require monitoring of web sites.
Additionally, huge sites like BoingBoing and Reddit are now sporting "STOP CENSORSHIP" logos.
SOPA is backed by the MPAA and RIAA.