Excellent question.  I don't have a definitive answer.  With some photos of unknown people in the collection of my husband's great-grandmother, I posted an inquiry on an Ancestry Message Board for the county in which she'd lived.  I included a link to the photos, which I'd posted on Flickr.  That was 8 years ago, and the photos have been viewed hundreds of times.  To my surprise, someone responded just last week, leaving comments for me on Flickr that identified many of the people in the photos, one of whom was her grandmother (as a child!). 

Perhaps this is an opportunity for someone to develop a better tool for searching/indexing large photo collections.

Ellen <lnmp@...>