I don't think it's down to the fans at all. There was always going to be rumours about him being appointed when it looked like Newcastle would finish the season managerless. Ashley made the decision because he thought it would lift the players and the club out of the relegation zone based on his status as former star and captain of the club. It hasn't worked. There is still time but it looks like an impossible task.
Ashley tried to sell the club because he fell out with the fans, I don't think Keegan / Shearer were appointments to try and do the team good. I think they were just to get the fans on side.
*btw Shearer could be a great manager, I just don't agree with managers walking in to jobs at a high level, go manager lower league teams, get some expirence.