You raise a very good point - but as I envision it the "Factions" concept wouldn't detract from this:

1. It would be optional, no one would be forced to participate or change in any way

2. people could use it as little or as much as they wanted - you could just be "loosely" affiliated with your faction, have no faction at all, or be a hardcore "factionite"

For me it would be kind of like my larger reality - 50% of the time I would spend in focussed discussions with "my" trusted users - not just to agree but to challenge each other within our faction / and then 50% I could spend just browsing and commenting in the public space we have currently.

There are actually quite a few subgroups on Dkos: the Food Group, the Energy Group (EnergizeAmerica), the edku group, TCers, PrettyBirdWomanHouse, etc. We usually set up on Google groups, or some like area. A lot of times it is just used to call the attention of group members to diaries or other articles/blogs/whatever that would be of interest to group members. Sometimes, like PrettyBirdWomanHouse, the group organizes actions to suppport the cause or, like the Food Group and EnergizeAmerica, organizes a panel for Netroots Nation.

While the information is more special-interest-specific, it isn't an "echo chamber" per se, since all of these are likely to have a lot of different viewpoints, even on a topic of common interest.