Dish eminence jeune Patrick Appel points out that internet users skew white, rich, educated, and young. Age is the greatest divide:

The young are more libertarian, pro-marijuana, and less religious than the American population generally. Millennials (pdf) are gay-friendly, racially tolerant, technologically savvy, welcoming of immigrants, open to government intervention, less hawkish, more accepting of non-traditional families, less inclined to marry early, and more optimistic about the state of the state of the nation. Thus, the consensus view among American Internet users may differ substantially from the result at the ballot box. This incongruity is amplified because senior citizens, the demographic least likely to have a robust online presence, has an outsized electoral footprint. 72 percent of American 65-to-74-year-olds voted in the 2008 election while only 48.5 percent of 18-to-24-year-olds exercised that right. We are in an era when the young have the most control over the dominant cultural medium while the old have the greatest say politically.

He also looks at the Iranian digital divide:

During last year's protests in Iran, Twitter was a primary means of receiving news from inside Iran. But how representative of the Iranian population were those tweeters? A few weeks after the protests broke out Sysomos found that 93 percent of Twitter users were located in Tehran, the center of the protests and one of Iranian opposition leader Mir-Hossein Mousavi's strongest bases of support.