what is the difference here between a face photo and DNA? I don't really see any. Both are kind of information fingerprint of a person. If police have a photo of a suspect is it ok for them to use image search of FB/Google/etc.?
What's the difference between this case and if the cops post a photo of a suspect and someone says "looks kinda like my co-worker, maybe a close relative" and that co-worker says "oh, that's my brother, we look alike, people mistake us for each other all the time." And then the DNA matches the brother.
People "unknowingly" give evidence to the police all the time. That's like complaining the police used information that someone blogged about to solve a crime.