Instagram is enjoying the biggest week in its existence. The company, already riding high on its iOS success, launched its Android app and captured 1 million downloads in its first day and unless you live under a rock, you will know that yesterday, Facebook came in and bought the company for a cool $1 billion.

Whilst everyone digests yesterday’s news, Instagram has just hit another impressive milestone — 5 million Android downloads in six days — making it one of the biggest Android app launches of all time. In fact, the iOS version of the app took about six months to reach 5 million users – on Android it took just six days.

Android users had been waiting a long time for the photo-sharing app to launch on the platform, and when even when the company launched its pre-registration process, smartphone owners rushed to be a part of it. When it opened pre-registration for its Android variant on March 24, it attracted 430,000 pre-registration signups in the following days.

Soon after launching on Google Play, Instagram was seeing 2,000 sign-ups a minute. The impact this had can be seen on the download graph below, which dipped shortly after launch as the initial demand subsided.

Today’s milestone is shown on Instagram’s Google Play page, which now registers downloads between 5 million and 10 million:

Screen Shot 2012 04 10 at 09.59.121 520x203 A million Instagram Android app downloads in a day isnt cool. You know whats cool? 5 million in 6 days.

We put the app through its paces to see how the Android flavor compared against the one that iPhone users have grown to know and love. We found that much of the aesthetic had been ported over from iOS, with just a few concessions to Android user interface conventions.

The news will be of equal importance to Facebook, which is seeking to push ahead with its mobile offerings, as it decides just how it will incorporate the service into its social network. For now, Instagram will operate as it always has but it’s a question of when, not if, the company will start to make changes.