I agree with those who suggest acknowledgement in newsletters or online venues. The more you can recognize someone's good work, the more good examples you put in front of others to contemplate. It is especially helpful to recognize people when they are among only one or two volunteers who turn out to contribute.
Get permission to post a photo to Facebook. One could use the "tag," "like" or "comment" features to amplify the gratitude.
Endorse someone's LinkedIn profile for volunteering or other relevant activity.
Your group's Web site
Telling the story of a volunteer is a perfect way to describe what your project does, how it benefits the community and who is involved. You don't have to write a big, full-blown article. Just cover the basics.
I scrolled through a number of Tweets about volunteering. Happily, the number of Tweets thanking volunteers was not very much different from those calling for volunteers.
Look at the following example of how much information one Tweet was able to provide about a volunteer and the help he has provided the community.
Mayfield schools honor 94-year-old volunteer, Pearl Harbor veteran http://t.co/szB2zwbHOv