Celina McPhail's mom wouldn't let her have a Facebook account. The 12-year-old is on Instagram instead.

Her mother, Maria McPhail, agreed to let her download the app onto her iPod Touch, because she thought she was fostering an interest in photography. But Ms. McPhail, of Austin, Texas, has learned that Celina and her friends mostly use the service to post and "like" Photoshopped photo-jokes and text messages they create on another free app called Versagram. When kids can't get on Facebook, "they're good at finding ways around...