This is the solution I used since this issue was making me crazier.
I use Stylish to customize Facebook, and I have a style installed that blocks all the useless crap on that site, i.e. friend recommendations, ads, unused navigation items on the left, etc.
All I did was find the DIV that holds the entire right sidebar and disabled it. This effectively got rid of the scrollbar and centers the main content within the middle DIV.
Here's the line I added to Stylish:
#rightCol {display:none;}
And just for your reference, here's the whole list I use for Facebook:
.rightColumnWrapper {display:none;}
#navHome{display:none;}
#pagelet_ego_pane {display:none;}
#pagelet_friend_list_suggestions {display:none;}
#appsNav {display:none;}
#pagelet_ticker {display:none;}
#fbSidebarGripper {display:none;}
#pagelet_welcome {display:none;}
#rightCol {display:none;}
.groupMemberSuggestionsList{display:none;}
.groupRecommendationsList{display:none;}
Hope this helps if you haven't already found a solution.