It appears Instagram may have passed the 25 million total users mark, if a bit of sleuthing around its API is to be believed.

The latest official numbers from Instagram were in December at the LeWeb conference, when CEO Kevin Systrom revealed that 15 million users had signed up. He told NPR later that month that he estimated the number had exceeded 16 million. Since then, there has been nothing official from the company, but a bit of digging reveals that the photo-sharing app’s growth appears to be continuing apace.

London-based Dirk Singer spotted, via a couple of Instagram apps – Statigram and Ink361 – that individual user numbers showed up in the apps’ user profile URLs. The numbers are consistent between the apps, indicating that they’re the unique numbers used by Instagram to identify each user.

Although there appears to be no number 1 or 2, account number three is Instagram CEO Kevin Systrom, with co-founder Mike Krieger at number 4.

Screen Shot 2012 03 02 at 14.22.52 520x370 Instagram appears to have passed 25 million users, adding up to 3000 more per hour

Now, it’s worth noting that private accounts, and accounts that presumably don’t exist, both show up marked as ‘private’. However, continue counting up and you can get almost to 26 million before the public accounts dry up. When Dirk checked four and a half hours ago at the time of writing, the highest account number he could find was 25,973,449. Checking just now, we reached 25,989,400 indicating that Instagram, assuming we’re counting accurately, could well have added over 3,000 accounts per hour in that time.

The danger of counting user ID numbers

Judging the number of users a service has by its account numbers is fraught with danger. Some users will have closed their accounts, and it’s fair to assume that a few may be in-house developer test accounts too, but seeing as the highest numbers are brand new users, with new accounts being added every few seconds, it seems that we’re probably onto something here.

Given that the highest user number right now is almost at 26 million, even with a generous margin for error, that’s safely 25 million. With estimates that the number of photos shared on Instagram are growing at twice the rate of Flickr, and the company targeting the Asian market with its latest update, the figure is unsurprising.

We’ve contacted Instagram for comment and will let you know if we hear back, but for now it certainly appears that it has passed the 25 million users mark.