We speculated several times in the past about social photo sharing service Instagram (which doesn’t have a website, it only comes as an iPhone app) to have reached and passed the 1 million users milestone, now it’s official. Instagram has gained more than a million users since its original release in early October.

As co-founder of Instagram Kevin Systrom told The New York Times:

Instagram, a social photo-sharing company that opened its shutters to iPhone owners just two months ago, announced Tuesday that it passed a major milestone of 1 million registered users.

“We’ve just been amazed at the growth of the service,” Mr. Systrom said in a phone interview. “My partner and I had a bet the first day about how many downloads we would get and I was off by an order magnitude.”

According to Mr. Systrom, Instagram users are “collectively uploading approximately three photos a second and tens of millions of photos have been shared on the service.”

The past two months have been a great run for Instagram, which launched its iPhone app with only a few selected users who had the privilege to try the app on board. Since then, usage of the service skyrocketed with the developers being busy keeping the servers up and running. The app reached another important milestone when Twitter announced support for Instagram photos in its inline media viewer on Twitter.com. The app also received a few updates to fix bugs and introduce more filters and Posterous integration.

Almost three months after launch and with “just an iPhone app” out in the wild, we think great things will come for Instagram in 2011. One million users is a solid foundation to start with.