I have recently started a class at my university, MIT, to introduce young students to the joy and benefits of social entrepreneurship. Students in the class will work on one of four projects: health and sanitation, education/ microfinance, technology, and charcoal. The site for the class is here: http://stellar.mit.edu/S/course/SP/sp09/SP.713/.I strongly encourage other university students to do the same. By participating in or starting classes that focus on social entrepreneurship, one can recruit many others to create the change we want to see in our local communities and global societies. 
For other social activists, I strongly encourage you to start community groups that address local and global problems and recruit passionate minds to solve these issues. 
Examples of social entrepreneurship groups: http://isites.harvard.edu/icb/icb.do?keyword=k3789​http://www.hcs.harvard.edu/~sec/
http://web.mit.edu/seid/