I started having trouble with Facebook on Safari a few months back, format changed to widescreen version which I presumed was for tablets (only the central column of FB was displayed, no access to sidebars etc - the only benefit I could see was that all adverts also disappeared).
I found workarounds for sharing, comments etc but copying/pasting became impossible as you have found.
Eventually found an older version of Firefox which seems to work for me (48.0.2 on Mac OS X 10.7.5). All facilities back to normal. Safari version was 6.1.6
Hope that's some use.
Ross