Kevin Systrom worked as an intern at Odeo (later creators of Twitter). He considered developing his own app for sharing pics and tagging users' locations. A check-in feature was already present in a few social media apps. However, those apps didn't offer photo sharing.
Although it was Burbn's advantage, it was still not enough to make its popularity boom. It had a complex UI in contrast to similar softwares, but users appreciated the photo-sharing feature. Systrom made the decision to delete all the other features and leave only that one. He started all over again.
Before launching, the developers shared Instagram with their friends, tested it extensively, and refined it to perfection. The main aspect of Instagram still was the picture-sharing feature. It was the key factor that worked for Instagram user acquisition, and made it instant success on the App Store.
On the first day, 25k people downloaded Instagram. Most probably, it was thanks to the fact that Systrom's pal and Twitter creator, Jack Dorsey, tried leveraging influencer marketing strategies. He got a nice number of followers on his Twitter profile. He tweeted about the app there, and it skyrocketed the Instagram user acquisition process. Many of his followers decided to take a closer look at the app.