I got the impression that nearly everyone on the panel felt that social media interaction was a necessary evil, yet also a valuable means to interact and hear from ones audience. Beware, however, of it becoming a distraction from the real work of writing.
Twitter seems to be the preferred method of communication, with feeds to other avenues such as Facebook.
They use Facebook as a "hub" to collect their content from other avenues (Twitter, blogs, etc.) and find that most of their feedback comes through Facebook and not through the original source.
Privacy was a concern regarding some of the social networking platforms, particularly Facebook and Google+. In general, Facebook and Google+ both got pretty negative feedback from the panel.
There was a general consensus to use social media to recommend, not to sell.
Cory Doctorow’s advice was to use the ones that you find satisfying.