Companies can Listen to what mumsnetters think about their products/services by reading the talk boards.
How do they engage? I have seen a reply from a Company Account on occasion but I would not call it engaging with the users, more like just a right to reply to a complaint.
Could be a thin line between engaging with users and advertising. If companies started conversations on talk boards, then that I would see as being advertising. If they joined in a discussion and gave their expertise without linking to their website, is that then advertising or engaging the community?
If Microsoft were allowed to post a reply to a How Do I Fix My Computer type post, that could be rather useful. Would it ever happen though?