So how does this relate to service learning? All ages need to learn to reflect about what they are doing in and outside the classroom, right? Voki is just another tool to encourage them to do so. You can give them a question about their service learning project, and they can respond through their avatar. They can change the voice, the look, the clothes, etc. This allows them to think about what their avatar will say. Each student's avatar can then talk to another student avatar. You now have created an internet class and deep discussions!
Any tool that encourages creative reflection and thought should be explored. Voki is an additional option to make your classroom a little more interesting and exciting. I suggest that you take a peek. Look around and see for yourself how fun it is to make your own avatar :0)