I already commented on Chris' initiative here, but here are some comments I made on David Crow's post:
Now, which of those listed platforms have portable data? Which of them are a suitable platform for building the (invariably) custom pieces that each community may want as they grow?
The tough part with many systems -- especially closed, hosted ones -- is that they provide great initial starting points, but then often lack in customization or growth options. And god forbid that your platform provider "go away" -- then you're completely stuck, and need to start over.
This is why I have chosen to go with fully open systems, because they can grow with communities and can never be locked down or disappear.
Facebook is ultimately closed and not a participant in the "open web". And it's someone else's platform with someone else's rules. I would hope that we steer around such closed instances and strive to connect openly.
"What I’d like to see is a platform that recognizes communities are networks of interests (people & organizations) and doesn’t try to become a destination"
I think this is spot on -- don't try and a become a destination IN AND OF ITSELF -- but rather add value through various aggregation and hub features. This also seems to argue for mini-networks that cross sites.
Open platforms are as important as open data or any other cross site initiatives. I don't care what you end up picking as your community platform, as long as your data and your users can seamlessly interwork with the other systems out there. The network is not the destination.