"Vast swaths of the Web effectively went dark" is a huge exaggeration.
Wikipedia is one of 2 sites (the other one I have never heard of) that completely went dark. Craigslist made people read through an opening letter before the reader looks down to "go to (regular craigslist)" which lets them search as normal. Then there is an excerpt on the screen about SOPA but that is easily ignored if you don't care. Google only blackened its name on the search engine but searching was unchanged.
If SOPA fails it probably fails for a number of reasons. The mode of protest by these sites was nothingness because it probably did not have a big impact on most people.
ETA: Wikipedia and the other site urged other sites to participate in the blackout but Google, facebook, and twitter said "helllll no we aren't losing money over this." LOL.
http://technolog.msnbc.msn.com/_news...9?preview=true