From the article:
Using social media and only the targets’ mug shots, a team competing in the
US State Department-sponsored Tag Challenge located three of five targeted
people in five cities in the US and Europe in less than 12 hours.
The Tag Challenge was significantly harder than the
Red Balloon Challenge because it required locating people
in highly populated cities, where the pace of life can help
people “hide in plain sight,”
and making it much more difficult to spot the suspects than to find large red balloons.
Furthermore, people are mobile, making it difficult to rule
out locations that have already been visited.