There has been plenty of talk about Facebook moving into the search space, and if there were a decent Internet search function integrated (with additional functionality for social media users) into the Facebook UI then it stands to reason that Google might get hit by whatever percentage of the billion FB'ers decided just to use that rather than jump out of their FB session.
Lots of ifs and buts and maybes, I admit. However, to state that it would be impossible for Facebook to kill Google is just as much of a stretch. Who thought that AltaVista would get killed, or Netscape for that matter. The felling of Internet giants is not without precedent.