Different audience, different topics. The LinkedIn group is more industry specific. For example a bike store owner may sell flatland products, but not ride. He would most likely not be a member here, but would fit in very well in an industry centric discussion. The LinkedIn group is not so much intended for "I need help with backpackers", as people who are involved also from a professional / career perspective. Of course there would be some overlap, but actually very different - assuming it takes off.