Perhaps you installed your /usr/bin/fab while running under Python 3? Check the 'shebang' line (#!/some/path/here) at the top of /usr/bin/fab, it may provide a clue.
If it's e.g. '/usr/bin/env python' then it should use the same python as 'which python' points to, but if it is something like '/usr/bin/python3', that would cause the behavior you're seeing.
If that doesn't clear it up you should let us know more details about your operating system version & how exactly you obtained/installed Fabric :)