UI
The user interface for a social experience is very different from the interface for an ecommerce experience. Ecommerce is easy; provide products and services alongside choices and community (many sizes, varying colors, customer reviews, and frequently-asked-questions) and you will sell til you run out of stock or project managers. Try to do that for the social experience and you will be writing code until the end of days.
When creating an interface for a social experience, you need to open your mind up a bit. Imagine a world without an end, then build it. That is what the social experience is; a world without an end. To build an interface for such a thing is folly. Or at the very least, an act of bravery requiring courageous effort and thick skin.
Start by reminding yourself every day that your user interface is going to be a continual work-in-progress. Each day users will dream up new ways to use your site and your interface needs to allow them to do so.
Think of a few of the actions users perform today: posting (blogs, message boards, music, video), communicating (commenting, emailing, messaging, tweeting), sharing (emailing, linking, messaging, tweeting). At a minimum, user interface for the social experience should include all of these.
Your user interface will need to allow users to perform all of these actions and most from a single web page.
Daunting? Of course it is.
Luckily, you don't need to reinvent the wheel just yet. Begin by looking at what others are doing today. Facebook, LinkedIn, Twitter, and so on down the list all have one thing in common. They've all spent millions researching and providing what they believe to be the best interface for the social experience. Draw upon their knowledge and apply it to your own.
You can be social and, in this day and age, you need to be.
User Experience
It can be argued that the social user experience is different from the ecommerce user experience, but at their cores, these two experiences are coming closer in line with each other as time goes by. Specifically, each is driven by page speed, logical presentation, and engagement.
Providing a social user interface is not the only step to take when providing a good user experience. You have to make your interface easy-to-use. Creating an account and logging in should be easy. Posting content or comments should be quick. Nothing about the experience should be slow or cumbersome. Everything should just "make sense".
A social user experience has to be fast. Web pages should load fast and posting content (posts, pictures, video) should be quick and painless.
A social user experience should have logical presentation. Navigation and menus should enhance the user experience and not overwhelm users. If navigation gets in the way of content, you have a problem.
A social site should be engaging. Users should feel like they are part of a community. Engage users by providing them with ways they may interact with your as well as other users. Message boards, comment sections, reviews, and other similar elements can help provide this engagement.
Social Interaction
Perhaps the most important piece of the social software design process, interaction is also one of the hardest. Interaction is very close to engagement. By engaging and interacting with your users you produce loyalty, something brands have been chasing for decades.
Almost as important as interacting with your users, you also need to provide a way for your users to interact with each other. You can't be there every minute of every day, so you need to provide a way for your users to take care of that interaction on their own. Your social user interface can help you provide for this interaction. Much of the interaction users engage in today is in the form of posting, communicating, and sharing. Your interface can be designed to provide your users with this type of interaction.
To be social, your site should have something your users can do together. If your site is an ecommerce site, users should be able to share information about purchases with others. Users should be able to suggest or review products and services. If your site is an informational site, users should be able to link to your information easily. Provide badges or widgets your users can share on their own sites and web pages.