Edited slightly from a recent post that I made:
The internet isn't going to end. The internet will change for the worse. There's a big difference.
Mostly-legitimate sites like YouTube and MegaUpload will arguably be hit the hardest. Their primary draw is the rampant copyright infringement. Users who upload original content will have to jump through hoops in order to validate that their content does not infringe anyone's copyright. There will probably be an initial month-long validation queue, which will eventually be streamlined down to a week-long wait. Some people will leave in protest, but most will just decry any dissent as "whining". In most likelihood, parody and other fair use exceptions will be suppressed, in the name of simplifying administrative duties. I predict the argument will go, "If we allow legitimate parodies through, then everyone will simply claim to be a subtle parody. Thus, our rule on no parodies, even if they are technically allowed, by law." In the end, YouTube will survive, but it will be nothing but insipid pet videos and hot, up-and-coming pop stars from major labels. Alternatives will pop up frequently on darknets, but none of the YouTube users will ever figure out how to access them. MegaUpload goes commercial, with no free accounts, in a move to pay for all the censorship that is required to clean up the site.
Quasi-legitimate sites, like 4chan, will either disappear or radically transform. My guess is that they'll all go underground. Anyone who can't figure out how to access them will be ridiculed as a lamer or noob. The government will swat at them, off and on, but nothing will ever really stick. A couple of them will simply move to European or Asian servers and abandon U.S. users. I have trouble imagining these sites going fully legit, but I guess stranger things have happened. In that case, full-time moderators would roam the boards, searching out any kind of copyright infringement and handing out frequent bans. After a while, the workload gets to be too much and the site closes down.
"Rogue" websites, such as piratebay, would be the first victims. They'll put up a token fight for a few months or years, but it won't go anywhere, and they'll all be forced to relocate to darknets or other various underground locales. Some will simply shrug and ignore the U.S. Again, the government will swat at them, and some of them will eventually be taken down, but new ones will simply pop up to replace them. Eventually, someone will be made an example of, with a 10-15 year prison sentence (if they're lucky). A show trial will briefly made the news, then be forgotten by all but the civil libertarians. A huge uproar on civil libertarian blogs will follow, along with further threads of "it's time for the ammo box!", but absolutely nothing will come of it, and they'll all stew in impotent anger. Slashdot follows every single fucking story with dogged perseverance, long after the mainstream media move on to other topics. In every single story, at least one person states, "If only you sheeple had voted for Ron Paul, none of this would have happened!", which becomes the newest Slashdot meme.
Controversial web sites, such as those espousing hate speech, expressing sympathy for terrorists (pro-Hamas or pro-Hezbollah), and right-wing militia groups will quickly be added to the lists. Most people won't miss them, but the civil libertarians will go berserk. A freedom of speech case will make it to the US Supreme Court, but nothing will come of it. In a 5-4 decision, the censorship will be upheld as constitutional. All the web sites move to European servers or darknets. The government halfheartedly swats at them off and on for the next ten years, until an example is made of someone, who probably ends up successfully fighting off the charges. It's hailed as a major win for civil libertarians, but nothing really changes, because nothing ever does. The government goes back to swatting halfheartedly at websites on darknets.
Sites like Flickr and Facebook, which generally depend on original content, rather than copyright infringement, escape unscathed. Uploading content becomes more tedious and annoying, as websites nonetheless attempt to cover their ass. More ads appear, as the sites attempt to pay for the additional time and effort necessary to patrol submissions by users. Users grumble, site administrators grumble, and everyone talks big about how they're going to vote out everyone who voted for SOPA. In the end, it's all just talk.
Most sites end up unaffected. A few end up blacklisted "accidentally". The end of the world is averted, and the internet goes on, like it always has... just in a way that's more authoritarian, less fun, and more annoying. Eventually, SOPA gets struck down by a liberal Supreme Court, and Slashdot goes berserk over something else that will surely cause the sky to fall.
I think SOPA is a horrible bill, and I hope it fails. The worst case scenario is frightening and admittedly could lead to some chilling effects. In fact, I think it's inevitable that many controversial sites (hate speech, for example) will end up being blacklisted, as well. However, to think that the internet, as a whole, will end up going down in flames seems simplistic and over-the-top.