This from Apple Support worked for me, whereas the other suggestions did not.
If Share button options (Email This Page, Facebook, etc.) or the Markup feature are missing after you install OS X Yosemite, follow the instructions below.
Follow these steps in each user account on your Mac to restore Share button options and the Markup feature.
- Open the Terminal app. You can launch Terminal from the Other folder in Launchpad, or enter "Terminal" in Spotlight search.
- Copy and paste this command into the Terminal window:/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/ LaunchServices.framework/Versions/A/Support/lsregister -kill -seed
- Press Return and wait for the Terminal prompt (ends in $) to reappear. It may take a few minutes.
- Close the Terminal app.
If you have another user account on your Mac that has this issue, log into that account and repeat these steps.