For a service that only exists on one platform, Instagram has been wildly successfully. The photo-sharing app for iOS is now on track to hit 15 million users, which as a post SocialFresh points out, is how many people are using Foursquare today.

Among mobile-first social services, Foursquare is arguably the biggest right now, but the geolocation check-in app is on track to be surpassed soon, despite being a year older than Instagram and being available on every major mobile platform and having a highly functional Web-based UI.

Instagram's growth has come exclusively on the heels of the iPhone's popularity, and received two boosts recently in the form of the launch of the iPhone 4S and being named Apple's iPhone App of the Year.

As SocialFresh writer Jason Keath points out, Instagram also benefits from the fact that its core functionality - taking, sharing and viewing photographs - are things we've done since the advent of consumer photography. Sharing our precise physical location with a network of others is an activity that's not quite as firmly embedded in our lives already.

Instagram For Android: Still in the Pipeline

Instagram's most recent project was a significant overhaul to the app's underlying functionality that went live in September. The company has long promised that the two items next on their list are a Web interface and and Android app.

It's that application for Android, which CEO Kevin Systrom recently confirmed they're currently building, that stands to send the service's user adoption rate through the roof. As beloved as the iPhone is, iOS still makes up a smaller share of the market than Android, which has itself ballooned pretty quickly. If the current buzz around the iOS version carries over across platforms, it wouldn't be unreasonable to expect Instagram's user base to double within a year or so.

The company's other top priority, a Web-based UI for the service, should also help attract users. Right now, people see Instagram shots posted to Facebook, Tumblr and Twitter all the time. When they click through, however, all they see is the photo itself, some basic information about the user who posted it and a link inviting them to download the iPhone app. For those who are unfamiliar with the service, there's very little detail about what Instagram is and why they should care about it. One way to get that idea across would be to offer the ability to explore more images from within the desktop browser.