It obviously replaced Snapchat but also Facebook, Tinder, even WhatsApp. What are you friends up to? Check your friends' stories. Wanna see some beautiful stories? Check some influencers' stories. Wanna tease a party or share fun pictures during a party, only with your friends? Use Instagram Stories. This app is a dream for people in their twenties.
The number of use cases this app created and the engagement it causes (the delay between posting a story and when it is seen by friends is rarely more than a few minutes) is really astonishing. Facebook will likely not die anytime soon, but Instagram is now the app and its acquisition was one of the smartest moves in the history of Facebook. If you want to learn more about the acquisition, I strongly recommend this article: https://www.vanityfair.com/news/business/2013/06/kara-swishe...