More late additions to the discussion!

I would use the term 'social' to describe any issue relating to groups of people within a society. For example, "...there are many social issues which affect the people of London".

I would use the term 'societal' when referring to the existence (or lack of) of 'a society' on the whole. For example "...this example of over exploitation of resources brings to mind the societal collapse of the Mayans".