I was going to suggest that they are not alternatives as such, rather that communism is a more focussed subset within the broader spectrum of socialist viewpoints. Speaking off the top of my head, I believe that socialism has more of a sense of 'the people' having a more direct involvement with the running of 'the state' for the mutual good, whereas in communism there is a more defined 'state' acting for the good of the 'people'.
Why are ex-communists socialists ? Well, I suppose everyone is supposed to move to the right with age.......