"Any Instagram user can add (or, tag) any Instagram account to their photos. An individual can now tag their photos with any other Instagram user, product, or the accounts of businesses. A business can do the same.... When Nike posts a photo of an athlete to its Instagram account, it can tag the photo so that the athlete can also add the photo to his or her stream. This engages Nike's followers as well as any followers of that athlete, increasing the overall number of views of the photo."
With service´s popularity comes the fame. Have you seen Nickelback´s fantastic parody video already. It really summarizes what the Instagram is all about, have fun!