I think that many Social Networkers have no clue about what "tagging a photo" means.
Tagging means labeling the people in the photo with their name.
That way, when you scroll your mouse across the picture, you can see who is who.
The right action to do otherwise is "sharing", not "tagging".
Otherwise what you're doing is marketing something out to other people's networks.
Which is going to get extremely annoying.