Wikipedia had a nice explanation of what is social trading.
Social trading is the process through which online financial investors rely on user generated financial content gathered from various Web 2.0 applications as the major information source for making financial trading decisions. Social trading introduces a new way of analyzing financial data by providing a ground to compare and copy trades, techniques and strategies. Until recently, investors and traders were relying on fundamental and technical analysis to form their investment decisions. Now investors and traders can integrate into their investment decision-process social indicators that are fuelled by a transparent real-time trading data-feed of all the users in the social trading network. This is now being introduced as social financial analysis. Social trading has also been associated with a variety of online social trading networks. These social trading networks can be considered a subcategory of online social networks.
Here is the link for more info: http://en.wikipedia.org/wiki/Social_trading