As you can see from the screenshot, the main Piictu UI is arranged around three tabs: Following are the conversations your friend have recently posted it, while Popular and Latest are the most popular and (surprise!) latest conversations across all users right now. By showing conversations across the service from users you don't yet follow, Piictu is trying to help you find new users to follow or interact with.
User discovery remains a tough problem to solve; the controversial Color app attempts to address the problem by using geographical closeness. Instagram has the Popular page (which in my opinion doesn't do a great job as it's mostly full of very samey pictures when I look at it) and, added in a recent update, the ability to see the photos my friends have liked and commented on (much more useful). Piictu also has a web interface and an Android client is in the works, so all this good-natured socialising isn't limited just to your iOS-toting friends.
Piictu also differs from the norm by not offering a range of cutesy filters to post-process your images. Depending on how you feel, this is either a handicap or a welcome breath of fresh air. Personally, despite being a keen user of Instagram, I have grown rather weary of its limited selection of heavy-handed special effects.
I now take my snaps in Camera+, which has a wider range of special effects with the ability to dial them down to be a bit more subtle (and can be used to great effect). Piictu of course supports this by allowing you to import pictures from the Camera Roll rather than taking them live.
All of this is wrapped up in an elegant and attractive UI. For example, look at the app logo and tab bar. As you scroll through the first quarter-screen of content, the logo scrolls out of view to maximise screen space -- but it scrolls at a slightly different rate so it feels more like it's automatically withdrawing than you're moving it. It's a subtle effect, and hard to describe, but its a very nice example of the app's thoughtful UI.
I spoke with Jonathan Slimak, one of Piictu's founders, about the vision for the software going forward. He told me that Piictu's built an international team (between nine of them, they span the Czech Republic, Venezuela, New York, and San Francisco). He said the project grew from a period during the last World Cup when their friends were using MMS to send pictures of themselves watching soccer games dressed in their various team's kits. They realised that there was a playful value in these pictures being sent to many people, but that MMS was a poor platform.
Piictu is designed to capture that sense of fun that Slimak and friends had with those MMSs, by using your pictures as elements in a dynamic, ongoing conversation. Clearly, these are lofty goals. It's undeniable that Piictu is entering a crowded market of image sharing apps for mobile devices and so might face an uphill struggle to get established with a critical mass of users.
However, I really feel that it has some interesting and novel ideas about how to do photo sharing and that those ideas warrant your attention. I urge you to download the app and take a look. In my considered, professional opinion as an app-reviewing scientician, it's five times more useful than the $41 million Color app, and I'm sure the Piictu team would be delighted with $205 million!