Having been around since the early days of identica/statusnet: no, not really.
It's always been small and a generally nice set of communities but it won't reach a large enough audience to matter. US (and to some degree) global politics are such a clusterfuck that it'll take more than some little servers with people discussing open source software and art or whatever to create substantial change.
that's not to say it's pointless. it's nice to have these little groups to focus on. it doesn't give me any hope but it does regulalry lift my spirits.
EDIT: I also dont think it needs to be any kind of agent or tool of change. Why can't it just be what it is? Groups of people socializing with one another?