Instagram is one of those iPhone apps that lots of people would talk about, but few people would actually use. Or at least that's what TechCrunch readers thought when the blog first covered the social photo sharing tool at launch nine months ago. Today, original reviewer MG Siegler reported back enthusiastically that not only are a lot of people using Instagram, the tiny startup is now one of the anointed Wunderkinds of the Apple mobile family, the kind that Steve Jobs features in the company's most blogged about keynote at WWDC. The size of the company versus the size of the userbase is astounding:

Yes, Instagram now has five million users. That’s 625,000 users for every month they’ve been in existence — with the growth accelerating. Just this past weekend they added 100,000 new users, for example. Even more amazing, there are now 1.25 million users for every one employee of Instagram.

That's only four employees and five million users. Kevin Systrom, the company's founder, told Siegler they're certainly interested in bringing a few more people on board but are in no huge rush. “Hiring great people is a top priority for me right now,” Systrom said. “We clearly have something special, and we want to make sure to have the best of the best to help us to the opportunity.”