Is this about co-design or not?
There's much in the Our Society proposal that looks promising but one thing fairly early on makes me wary. "It should be a social enterprise,..."
Alternatively it might include social enterprise, but surely not everyone who has something to offer Our Society will be comfortable with a social enterprise (throughout) environment? Tessy Britton refers to "a *possible* danger of it being perceived as My Big Society rather than the Our Society they clearly want to create, in other words, another labeled house where not everyone will feel comfortable?" 
If it's intended that Our Society should be about co-production, then this involves co-design. Proposing how things might be is fine, but isn't it simply unneccessary to rule some people out by a small group at the start plumping for a particular business model?
A comment about platforms and networks
To borrow or build on a phrase used by Julian in an earlier comment , the idea of one hub to rule them all has probably passed its sell by date. One network which seems to be exploring an alternative is the Transition network (for transition towns). If I'm understanding it right (had a minor involvement with the design process ) the network realise its just one of many players in a diverse landscape, so it doesn't seek to dominate. But this doesn't preclude them designing increasingly sophisticated ways of connecting, aggregating and sharing content from the wider transition towns ecosystem... which can help with the "exposing and collaborative structures ... platforms and bridges and mosaics.... " that Tessy Britton refers to.
The implication of this for something like Our Society is that it could be fairly relaxed about being one player amongst many. So if for example I thought it useful to set up a "Our sustainable society" group on say WiserEarth, this is not something I should need anyone's permissison for. That's right isn't it Julian? (Sorry Tessy, and I may be misunderstanding, but my heart sinks if and when anyone starts refering to ideas like this as a brand).
Values and transparency
It's probably been said many times how one of the most useful things that can be done in starting something like Our Society is to co-create a shared understanding of the values it will seek to follow and practice. For example there can be multiple shared (mis)understandings of the simple word 'Sharing'. A website which 'shares' content whilst saying 'All rights reserved' has a very different feel from one which uses a creative commons licence.
Just my twopenny worth, but transparency seems to me to be one of the most crucial values. I must admit to being a little wary, even if I'm just plain ignorant, of how total transparency sits entirely comfortably with any business model, even if social enterprise. This being another reason for a not-fracturing, but simply realistic recognition of the value of diversity across any potential network.