Kevin Systrom interned at Odeo (a company that made Twitter sometime later). He thought about making his own app for photo-sharing and location tagging. There were some social network apps with the checking-in feature. But, they didn't mix it with sharing photos.
Burn was unique in this, but it wasn't enough to make it popular. Its UI was too complex in comparison to similar apps, but people liked the photo-sharing feature. Systrom decided to remove everything but photo-sharing. He started from scratch.
The creators shared Instagram with friends, tested it a lot, and polished it to perfection before launching. Instagram's key feature was photo-sharing. It became an instant hit on App Store.
25k users downloaded Instagram on the first day. It's probably because Jack Dorsey, the founder of Twitter, was a friend of Systrom, and twitted about Instagram to his IM followers. Many of them checked it out.
