Well, other than starting a way to donate money to certain causes, we could start a group specifically for calls to action to volunteer time locally. Animal shelters are good ones, especially those that follow a no-kill policy and obviously at homeless shelters or kitchens are pretty common.
The individual users could post about efforts they want to organize in a call to action of sorts and hopefully people will answer. Of course, this works better in some locations than others. They could then document it and share their experiences, what they've learned, what works and doesn't to help improve the process, etc.