There is value and logic in using a single platform. E.g. in the area of the Green Knowledge Trust, Appropedia is actually the product of mergers between several wikis.
However, the idea of a single platform has some big drawbacks.
Who decides which platform? We have no benevolent dictator for the entire climate/environmental movement. If COTW says “Use X,” some will and some won’t.
Communities often work better at a smaller size. Note Dunbar’s number of roughly 150 humans to a social group. (Larger groups can be sensitive to this by allowing for various ways of grouping, formally and informally, within the larger community.)
Some people just will not join “the one platform” or “the one site” on principle.
Others will think that another site is better, and be active there instead.
Others will object on principle to having no choice on this particular issue, even if they register with the one site. Discontent is not helpful to our efforts (and leaving people disempowered and unhapppy is basically not a good thing).
“One site to rule them all” or a perception of an online “Borg” invites a counter-reaction.
But it doesn’t have to be a big problem. The vision we’ve been talking about in the Coalition of the Willing is to let sites be interoperable, with a minimal number of sturdy and easy-to-follow open protocols. This allows for a much more complete participation. We already see this working with OpenStatus tying together Identica/StatusNet and other sites. (It’s possible to follow someone’s Buzz account plus profiles on different StatusNet installations from a StatusNet site such as Identica. They are leading the development of the OStatus procotol, ironing out the wrinkles.)
It seems likely that there will be a single “main” site for, say, the Catalyst Network or Green Knowledge Trust, but it will be chosen by the swarm. And they can move any time, and (like with the Diaspora platform) they can take their friends with them.