Instagram has had quite the few weeks, from moving away from an iOS-only app by releasing a version for Android, to then being bought by Facebook. If there was any doubt before that Instagram was an example of a wildly successful mobile app, there definitely isn't anymore. But what really got some talking is how the service became so successful with basically no website.
Without a doubt, this strategy cannot and will not work for all apps. But here are things that anyone with a mobile app should consider:
When and where will people want to use your app?
Instagram's mobile-only strategy worked because people take pictures on the go. This is not as true for viewing photos, which was remedied with the sharing capabilities. By allowing sharing to social networks as well as assigning a URL to each photo that could be viewed on a computer, the photos had a reach beyond the app's users alone. This is an excellent marketing strategy as well, because the photos stood out compared to "no filter" pictures that make up most of the shared photos on social networks.
However, some types of apps simply cannot be limited to mobile devices only. I believe productivity apps, in particular, need strong web companions. Apps such as the note-taking app Evernote are great for jotting things down on the go. But people using Evernote frequently use it for getting work done. How productive would it be if while doing work on your computer, you had to constantly refer to your phone?
How much simplicity can we get away with?
Instagram focuses on simplicity: a simple interface, and a simple process. You can upload a photo to Instagram, choose a filter, write a caption, and share it to other social networks in just a few seconds, given you're not as indecisive as I am when it comes to choosing a filter.
But once again, this approach can fail. While simple interfaces can be beautiful, it is more important to guarantee that the user can easily do anything they would want to be able to do with your app. Never sacrifice aspects of the experience in exchange for a better-looking app.
How much information do users need about your app?
I'm not going to name names, but I hate when you need to download an app just to figure out what it actually does. While it's one thing for your app's features to be limited to mobile-only, it's another to limit the information. Sure, you can find a blog post or review of almost any app, but people will still want more information. Nothing is worse than downloading an app that does not have a 'Help' or 'FAQ' web page, especially when you are looking for help about how to use the app. If a user doesn't know how to use the app, how are they going to figure out how to get to the 'Help' area of the app? They might just delete it.