Intrinsic Value
Most people aren't professional photographers. They don't have access to the tools professional photographers use nor the time or money to buy the tools and learn them. But they do want professional looking photos. Instagram built a platform that had intrinsic value on it's own by providing people the ability to easily create professional looking photographs.
Simplification
One of the keys that made Instagram successful was that they took something people were already doing and made it easier and better. They removed the need to download photos from a mobile device and then upload them from a desktop or laptop. They simplified the process and allowed users to share exiting photos right from their phones or take a new photo to share.
They also simplified an otherwise complex photo editing process by providing a set of templates that people could apply without having to understand image editing. This enabled a regular person to have professional looking photos without any work.
Interoperability
One thing that Instagram did very well is that it built it's software to work with exiting services like Facebook and Twitter. Services that already had a huge community of users. They didn't try to re-create Twitter or Facebook. Instead, they allowed Twitter and Facebook to get better by making their service interoperable.
Choice
Instagram didn't force people to ONLY use their service. Instead, it allowed people to continue to use their existing social media services and augment a specific workflow to be better if they wanted to. Users don't want to be asked to chose between this OR that. They want to be able to use this AND that. Will your software provide additional choices or limit choices?