Instagram, the poster child of the mobile photo-sharing craze, now has more than 5 million users.

The eight month-old iPhone app added 100,000 new users this past weekend and is now seeing nearly 860,000 photos posted to its service each day, co-founder and CEO Kevin Systrom shared with TechCrunch.

Collectively, Instagram users have already posted more than 95 million photos, which means the startup should cross the 100 million photo mark before the end of the week.

Part of Instagram's early success can be attributed to a still nascent but very popular API that's now being accessed by more than 2,500 applications. Webstagram and Flipboard are among the most popular, said Systrom.

We've been tracking Instagram's insta-comeuppance. You may recall that the app hit 100,000 users in week one, 1 million users in ten weeks and just recently surpassed 4.25 million users in late May.

Lately, we've been intrigued by top notch third-party Instagram applications (as seen below), inspired by the fashion photographers flocking to the application and excited by the potential opportunities for early adopter bands.

13 Instagram Apps for More Photo Fun