Geoff Mulgan is the CEO of NESTA in the UK (National Endowment on the Sciences, Technology and the Arts). Geoff's newest and preferred definition of social innovation is "innovation which is social in both its goals and its means".
I like this formulation. It requires social means (relationships, exchanges, coalitions, transfers of information and power) between social actors (organisations, individuals, governments) to achieve social goals (social well-being and social change).