Viewfinder (free, with some in-app purchase options) is a bit like the Photo app in iOS, but on steroids. With your permission, It takes your image library and arranges it by time and location. All photos remain private and only you can see them unless you choose to share them. Nothing is uploaded to the cloud, although the app requires a free account. Photos you share with others can be shared by them to others, so keep that in mind before you use the sharing options.
You can import contacts from your phone, from Gmail, and Facebook. Once contacts have been imported, Viewfinder shows you which friends have also set up an account on Viewfinder. In-line messaging and commenting are supported for creating communities around your photos. None of this sharing is done online -- everything stays within the app, except the communications out and back that is managed by Viewfinder.
The app gives each user 1 GB of storage, and the developers are happy to sell you more. Viewfinder Plus (5 GB of storage) is US$1.99 a month. Viewfinder Pro (50 GB) is $9.99 a month. In practice, most people won't use either paid option.
Some of the features found in Viewfinder will be in iOS 7 when it comes out later this year, in particular the better organization of photos by date and location. iOS 7 will even separate panoramas into a specific album. Of course Apple has the Photo Stream feature now, but I find it to be cumbersome to use when trying to share images with others.
Viewfinder is not a universal app, and I tested it on an iPhone running iOS 6. It worked as expected with no issues. It failed on iOS 7, and even helpfully popped up a note saying that iOS 7 was not supported yet.
Viewfinder is a nice app with appropriate privacy and the ability to build photo communities. It does have some feature overlap with iOS 7, but it also performs some functions that won't likely be part of iOS 7. The app requires iOS 5 or later, and is optimized for the iPhone 5.