Malcaolm Gladwell recently wrote a thought-provoking article "Why the revolution will not be tweeted". His argument is that Twitter (and other social media) are good at "the weak tie" -
"The platforms of social media are built around weak ties. Twitter is a way of following (or being followed by) people you may never have met. Facebook is a tool for efficiently managing your acquaintances, for keeping up with the people you would not otherwise be able to stay in touch with. That’s why you can have a thousand “friends” on Facebook, as you never could in real life.
This is in many ways a wonderful thing. There is strength in weak ties, as the sociologist Mark Granovetter has observed. Our acquaintances—not our friends—are our greatest source of new ideas and information. The Internet lets us exploit the power of these kinds of distant connections with marvellous efficiency. It’s terrific at the diffusion of innovation, interdisciplinary collaboration, seamlessly matching up buyers and sellers, and the logistical functions of the dating world. But weak ties seldom lead to high-risk activism."
His example of "high-risk activism" is involvement in the US Civil Rights movement - activists ran a substantial risk of threats, abuse and violence.
The article has been the source of several rebuttals - by Chris Lake on eConsultancy and by Leo Mirani in the Guardian, for example, but these authors seem not to be disagreeing with Malcolm Gladwell as much as they might seem to be - Mr Gladwell does not (as you can see from the quote above) say that social media are ineffective or useless, just that they are only good for certain things (and potentially VERY good at those).
What blurs this further is that the people who follow your tweets or are your facebook friends do quite likely include those who most passionately and unconditionally wish you or your cause success. But they are probably right up the top of the Zipf curve, greatly outnumbered by people to whom you are not massively important (at least not yet). Zipf curves seem very common in volunteer- or activist-powered areas: you get a Zipf curve when a small number of people contribute a lot, a larger number contribute some, and most contribute hardly at all - here's an earlier Usability Notes post on the zipf curve and user-generated sites.
There are many cases where getting many to do a little is more effective than getting few to do a lot. As a humble example, take the Oxford Oxfam Group street collection for the victims of the January 2010 earhtquake in Haiti (I'm picking this example because I'm currently the Chair of the Oxford Oxfam Group). Facebook was notably successful as one means of spreading the word that we needed collectors. Standing on a Oxford street corner for some hours on a winter Saturday strikes me as a reasonable-sized ask: it's cold and boring, though of course well worthwhile for the money collected. But, let's face it, it's most unlikely to risk intimidation, abuse or violence as Civil Rights campaigners did.