First, they have the potential to facilitate incidental exposure, in other words they can reach non-traditional audiences with content about science in an online space where they are not otherwise looking for it. Facebook takes on the big problems of choice and audience preference gaps in reaching the wider public.
Second, users with an interest in and enthusiasm for science can serve as "science navigators" or opinion-leaders, passing on information to friends about new science-related events or issues in science, recruiting friends to participate in science-related activities or campaigns.
There is already a fertile network of "Common Interest-Science" groups on Facebook. And there is untapped potential for cross-cutting ties between these members and other content areas such as Politics, Current Events, Philosophy, Beliefs & Causes, and Religion & Spirituality. (Indeed, sociologists who study boundary work could have a field day studying how the arbitrary lines we often draw in the "real world" between institutions and areas of knowledge are blurred at social networking sites.)
So here's an idea: How about official Facebook pages and groups for the Exploratorium, 2009: The Year of Science, NASA, AAAS, the National Academies or other scientific organizations?