Nick,

This is my cousin Marcus with his new French fry earmuffs. He also looks a lot like Russell from Up.

Anyway, good question! It’s remarkable that Instagram can attract a million users in three months — a number that took Twitter two years to hit. But what explains Instagram fever?

I’ll be honest: I’m not quite sure. It’s a simple idea, almost perfectly executed, but sharing iPhone photos is definitely not an original idea.

Path, which launched a month after Instagram, is nearly the same app. Both are photo-sharing social networks built exclusively for the iPhone (for now at least), but if I saw the two services side by side six months ago, I would’ve pegged Path as the winner. It’s founded by an early Facebook big wig Dave Morin and advised by Shawn Fanning (who you might remember as the Napster guy who isn’t Justin Timberlake), along with a team of 14 (as opposed to Instagram’s two) and a decent amount of startup capital. Add to that Path’s tech blog hype and heartstring-tugging promo video.

(Actually, a word on that video: Around the time Path was released, my girlfriend Megan pointed out that we had very few couple photos. I didn’t really see the significance of having pictures, but I’ll admit that Path’s “Nervous at Home” video convinced me otherwise.)

But there are some small but important differences between Path and Instagram, the biggest one being the relationship between users. Path, like Facebook, requires a mutual acknowledgement for friend requests. (In fact, the friend limit of 50 was based on the evolutionary theory that 150 is the maximum number of social relationships that a human can maintain.) On the other hand, Instagram uses the Twitter model of followers, where you can follow a user whereas they might not follow you back. It’s friend vs. follow; private vs. public.

I would’ve expected that something as personal as photos would do better in a private setting, but it certainly underestimates how badly people like to share things on Twitter, Facebook, email, etc. And that’s likely been a major stumbling block to Path’s growth.

There’s also one other major difference between Path and Instagram: the filters. Path doesn’t give you any way to edit your photos, so if you have an older iPhone with a mediocre camera, like I do, then you’re stuck sharing mediocre photos. The hyper-saturated colors Instagram and similar photo apps are a good way to make low-quality, smartphone photos look artsy.

I’d like to think the wild success of Instagram can be attributed to more than Hipstamatic-y photo effects, but am I underestimating just how much people like them? In all fairness, my favorite photo of me and Megan was taken in Hipstamatic.

Hey Kevin,

Check out my cat.

Wait, is that a totally uninteresting image from my life, gussied up with a gratuitous sepia filter? That’s right! It’s time to talk about everyone’s favorite new social mobile hipster vintage photo sharing service, Instagram!

I first downloaded the iPhone app a couple months ago, when Gruber posted it. It seemed well made, but neither he nor I could see the point in it. At the time, he said, “The app is nice, but I can’t see why I’d use the sharing service instead of Flickr.” That made all the sense in the world to me. I deleted the app before sharing a single photo.

Fast forward to last week, when Instagram announced it had surpassed one million users. That’s astonishing growth for a social network that only works on one mobile platform.

What’s going on here? I’m perplexed, but then again, I’m the guy who suggested we do an entire week on VYou, a site which now seems pretty doomed. I’m clearly missing a piece of the puzzle when it comes to evaluating new social networks. So, what makes Instagram such a hit?