"Social Media is a party, Brands are the narcs in the corner taking notes"
Martin Green from Meebo makes the point that this decision is entirely contextual. Depending upon the "intimacy" of a conversation, it may be appropriate for a brand to drop in.
So we're all agreeing that ignoring the conversation is worst practices. Listen, but carefully, responsibly and with disclosure. OK, now what? More listening? Still feels like the narc in the corner to me. How about you?