I often rail against the gratuitous use of social media icons on Ecommerce sites. It takes people away from your site for little real benefit. The folks at Monetate do a great job of summarizing the best way to use social for the good of your business and of your visitors.
How a visitor interacts with your site is important to conversion. Here is an analysis of the identities used to login to news, business, entertainment, and other sites
In my new book Your Customer Creation Equation I discuss the importance of getting new users of Online Services to return and login. This process is called "on boarding" and it is the process of making your new users experts at using your service. Wunderlist is how me and my wife share grocery lists. and their on-boarding process is very tight. Here's how they do it.