When the American Red Cross was able to raise $200K+ via Facebook Causes in a matter of days for Japan, I was impressed but honestly not totally surprised. When the pool of people you are talking to is that big and the platform you are talking on is innately social, your ability to get a group of people to take an action is fairly likely. (Not to mention Japan relief efforts being a very worthy cause). Much in the same way that we take cell phones for granted and even the mobile web, I think the ability to connect millions of people in an instant has become the norm. I also think that's a good thing. From political uprisings to connecting people in natural disasters social has become a tool we can't live without.
And the power of this connection is built for making movements - whether that be a movement to provide disaster relief or something much lighter like getting Oreo into the Guinness Book of World Records for the most likes on a FB post.
The key to starting a movement on social is to ask people to take small, easy actions, because on social even a baby step can lead to fairly big pay-offs. Obama asked his followers to donate $25 and raised millions. The Red Cross took the same approach with Japan. They were also smart enough to realize the value of world of mouth - if you didn't want to donate you could still participate by acting as a "promoter" and getting the word out by donating your status update.
Brand movements should also ask little from their participants, knowing they'll reap much more in return. A single like of a post can result in someone's 200 plus friends seeing your brand.
Some may complain that social has infringed on privacy or caused us to rely on "fake" online connections versus making real friends, but I think social is uniting us in a way that is extremely powerful. I'm excited to sit back and watch what other good deeds we accomplish as one big social community.