Omoto and Snyder (1995) is only a single study on volunteerism with a sample of 116 AIDS volunteers, but the results are quite interesting nonetheless. In a Snyder, Omoto and Crain (1999) they summarize:

Motivations [..] foreshadow the length of time that volunteers stay active (Omoto and Snyder, 1995). In one longitudinal study, volunteers who were more motivated […] when they began their work were more likely to still be active 2.5 years later. Interestingly, relatively self-focused motivations (i.e., personal development, understanding, esteem enhancement) were more predictive of volunteers’ duration of service than those that were more other-focused (i.e., values and beliefs, community concern). That is, volunteers remained active to the extent that they more strongly endorsed relatively self-focused motivations for their work. Other-focused motives, even though they may provide considerable impetus for people to become volunteers, may not sustain volunteers faced with the tough realities and personal costs of volunteering.

The study also has other interesting results. For instance, “90% of respondents expected to continue volunteering with the agency for at least another year. In actuality, 54% of the volunteers were still active 1 year later, whereas only 16% of them were still active 2.5 years later.” (Omoto and Snyder 1995, p. 677)

I haven’t looked into the literature much more but this seems to be exactly the kind of research one should turn to if I wanted to design a successful social movement.