According to Wikipedia:
SOPA and PIPA would put the burden on website owners to police user-contributed material and call for the unnecessary blocking of entire sites. Small sites won't have sufficient resources to defend themselves. Big media companies may seek to cut off funding sources for their foreign competitors, even if copyright isn't being infringed. Foreign sites will be blacklisted, which means they won't show up in major search engines. SOPA and PIPA would build a framework for future restrictions and suppression.
You can also use Wikipedia's blackout page to find your representatives by zip code. Contact them and let them know your concerns.