David Weinberger said something great about the allegation that online political conversations create an “echo chamber.”
Some of the time you want to talk to people who are different and learn from them. Some of the time you want to find supporters, energize supporters, and acheive a goal. You don’t have to do both things at the same time.
When you’re learning and coalition-building, it’s good talk to everyone, find and create common ground. When you’re trying to achieve the goal, you find allies, co-ordinate with them, cheer with them.