This morning a few of the share menu itmes were missing from Safari on macOS Sierra. Looking in the Settings app I noticed that the menu item had been disabled. Clicking the enable check did nothing. I was not able to enable the share menu item for Facebook, Twitter, etc. I found this link with the following command that repaired the issue.

/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -seed