At the end of the day the younger internet using population that you see on sites like Hubski and Reddit only account for so much of the population. It may seem very large while you're on those sites, but it's more an issue of localization. In reality the people are spread out few and far between, and outside of major cities you're not going to see people as up to date on technology and in arms about politics geared towards technology/science/etc.
That said, you can't expect to apply the discussion and uproar on the internet about a candidate to a real election, at least not yet.
This article is pretty worthless. Congressional races are notoriously uncompetitive. Young people can't be counted on to vote.
At the end of the day, the voters of Texas-21 simply didn't care about SOPA as much as the internet did, and reelected their perfectly decent (in their eyes) representative.