if you uploaded a photo with a friend in it, you could “tag” them and they would receive a notification that you had posted a photo of them online. Facebook Photos took off right away. Within three weeks, Facebook hosted more photos than Flickr.50 After a month, 85% of the service’s users had been tagged in at least one photo.51 Zuckerberg and the rest of the team were amazed that an arguably inferior product could so quickly unseat the incumbents. The secret sauce had to be the network effects.
