Allow me to inject a cynical slant to your hopeful suggestion that people 'today more than ever' can chose their peers. People with a certain education supported by an infrastructure (personal and technological) that allows for choice have this flexibility, but those without see only the threat of 'the other,' not an opportunity to rethink fundamental principles. Worse, political leaders continue to exploit ignorance, but with sophisticated mass communication tools and technologies Twain could never have imagined.

Prominent examples:

1. Lower-middle class Southern whites in the U.S. (with whom Twain was intimately familiar) embrace right-wing rhetoric (devout opposition to a godless, liberal govt that rewards a free-loading underclass) while the Republican Party caves to the self-interest of the mega-corp oligarchy. These folks who would benefit from social reforms reject all such liberal notions because, well, they still believe anyone who's not white and Christian is out to take away what little they have. But regressive conservative tax policy is actually doing them harm. (tl;dr: see the success of Fox News, with its overwhelmingly white/old audience. http://www.forbes.com/sites/greatspeculations/2015/04/02/rat...)

2. Given the above in the U.S., is it any wonder radical Islam succeeds in less affluent, less educated areas of the world--where there's very little exposure to outside ideas. And there's certainly very little opportunity for social/intellectual/economic mobility.