I'm not sure this has as much to do with her age as her gender (although I'm sure the former does play some kind of a role). My sister uses snapchat, instagram, and facetime all the time in the same ways as you described your sister does, and she's graduated from college. She uses it predominantly to communicate with her other female friends.
I also know several girls my age (25/26) who use these services in the same way. It tends to be mostly the "social butterfly" type of girls. It's funny because these are the same girls who would flood their facebook walls with pictures throughout the day, and now they rarely if ever make a post. I could be wrong, but I don't see any guys using the services in the same way.