I chaired a session on this at the 2009 ARNOVA Conference in Cleveland. There was a lot of excellent discussion and spirited debate.
During the 2008 Presidential Election, much was made of the community organization experience of then candidate Barack Obama. This combined with interest about the Obama Campaign's use of on-line organizing and interest by younger voters in working for causes. The reaction of the traditional community organization sector wasn't completely positive however. For example (far from the only one), Sally Cohen of the Center for Community Change wrote an opinion article in the Christian Science Monitor arguing that if young people want to make real change they will have to do it in face to face communities. She provide a set of examples from US History and concludes that on-line efforts are individualistic efforts. Her concerns have been echoed by others in the traditional social change community.
In the first place I think this is an unfair characterization of how on-line techniques work. There is definitely a social dimension to current network technologies--people work together using technology to solve problems. That is certainly different from the activism of earlier days but it doesn't mean that it is less effective.
If everything we did in the 1960s and 1970s had work, wouldn't we be living in a very different society? I do not mean to disparage the efforts of those who use traditional social change techniques (and I'm often one of them) nor to suggest that these methods are ineffective. In point of fact, however, there really isn't a boatload of credible scientific evidence that supports much of what traditional social change practitioners do. That doesn't mean that it doesn't work--only that we don't know. At the same time, there is a beginning body of evidence that on-line techniques can be effective.
To be honest, I wonder how many traditional techniques have survived beyond the point where their underlying assumptions about society are valid. Some of the ideas that might have been relevant in the 20's and 30's or even the 60's and 70's are now facing a society and an economy that is almost completely different. Even in the 35 years I've been in the field things have changed radically.
At the end of the day, I suspect that we might need a mix for many situations, on-line efforts for others and face to face for still others. We need better knowledge about which things work and where. This is too important to rely only on assumptions.