New means of communication--the Net, but also direct mail, cable television, phone trees, and so on--have encouraged small inbred groups that reinforce their own opinions.
But this capture of the public realm by private groups and the consequent breakup of a public agora into micro-clubs that listen almost exclusively to themselves -- this has been going on at least since the time of Athenian democracy. In most of history so-called public discourse has been either monopolized by ogliopolies or divided by the oppositional relations of small groups.
This doesn't lessen the need for a true public space, but it cautions us not to expect miracles.