An article describes the issue:
For example if you post a link to the story on your Facebook wall. Under SOPA, all of Facebook can be blocked. To avoid this fate, Facebook would be responsible for policing the copyright status of every piece of content its users post. The same happens with search engines, which to avoid being shut down, Google and Bing would be responsible for policing the copyright ownership of every piece of content they index.That includes Blogger too, of course: we could easily find ourselves on the wrong side of this, simply because some other blog on Blogger posted something that was copywritten. How likely is that? I'd have to say the probability approaches certainty.