It was only this past December when Instagram claimed to have 15 million users. A mere three months later, the number of people out in the world using the app to take digitally touched-up photos appears to have ballooned to 25 million. The number is unofficial, though, as no confirmation of the milestone has come from Instagram as of yet.

Dirk Singer, blogger of social media stats and research, came upon Instagram’s likely achievement when he noticed, through the use of two Instagram web viewers, that the individual account numbers assigned to new users appears to have exceeded 25,000,000. The account numbers appear to be sequenced to users based on when they signed up for the service. For instance, Singer points out, Instagram CEO Kevin Systrom is 3. An Instagram engineer is 20. The user ID sequencing continues longitudinally until, skipping ahead to today, Instagram user 25,973,449 signed up this morning around 10AM GMT.

While gingerly concurring with Singer’s find, TheNextWeb cautions that quantifying the number of users of a service based solely on the assigned account numbers “is fraught with danger.” As they note, there’s no knowing how many users have closed their accounts, which would obviously lessen the total of actual users of Instagram. However, TheNextWeb does agree with the speculation and corroborates the 25M estimate by pointing out that the service obtained nearly 3,000 new accounts per hour during the time in which they were verifying Singer’s discovery.

In truth, not only is that a lot of users (it’s more than foursquare, Singer points out), but that’s an unfathomable amount of arty-farty pictures people are passing around on their smartphones and tablets.

UPDATE!: Dirk Singer, the statistics sleuth who broke the story about Instagram’s 25 million user milestone, commented with the following affirmation:

Since publishing my post, I am more certain that the 25/26 million figure is right. It’s possible to test this by matching user IDs against Instagram’s own pronouncements.

As a result the methodology of using user IDs via the Instagram API seems to be the one Instagram uses itself to figure out user numbers. Of course – I completely accept that 25 million users is not the same as 25 million unique users!

So you can hedge your bets on this one – Instagram’s definitely crossed over 25 million.