Instagram has topped 25 million users, becoming the largest mobile-based social network in the world, according to a report.

Dirk Singer, the head of U.K. social media agency Rabbit, claims on his Lies, Damned Lies & Statistics blog that the maker of the popular iPhone photo app just recently surpassed that figure, while Foursquare is at 22 million users. Singer based his analysis on two Instagram web viewers, Statigram and Ink361, both of which use Instagram's API.

Singer noticed that the URLs on the sites reveal user numbers. For instance, Kevin Systrom, CEO and co-founder of Instagram, is apparently user number three:

Singer cited user @holliejsmith's user number at 10 a.m. GMT was 25,973,449 on both viewers, meaning Instagram was close to 26 million users. That's a big jump for the service, which claimed 16 million users in December.

Meanwhile, using the same reasoning, Singer estimates that Foursquare is now at around 22 million users.

As The Next Web notes, reading URLs is far from a definitive way to get a read on Instagram or Foursquare's actual numbers. Some accounts may be closed, the site notes, while others may be merely test accounts. Moreover, there's no "number one" or "number two" account.

Reps from Instagram could not be reached for comment on the report.

Meanwhile, Instagram's growth is set to skyrocket soon as the company prepares an Android app, which should be hitting any day now.

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