As you know, the Model Rules were amended recently to recognize that an element of competence is knowledge about "technology" which includes computer programs and the use of social media. In addition, some bar groups have also issued reports or opinions on social media. For example, I recently reported that the Philadelphia Bar Association’s Professional Guidance Committee issued an opinion on the ethics of advising clients on the use of social media.
But "social media" includes so many different things and its use can have so many implications that more guidelines are needed. Fortunately (at least for lawyers in New York), earlier this year, the New York State Bar Association issued a report called "Social Media Ethics Guidelines" (available here.) The guidelines discuss topics ranging from attorney advertising and furnishing legal advice through social media to ethically communicating with clients and researching prospective and sitting jurors. Needless to say, these and other guidelines will continue to evolve in order to keep pace with technological developments.