After upgrading to OS X El Capitan on iMac in Photos the share menu does not include Mail or Facebook so cannot share photos from within app

Make sure, Mail has been enabled as the default Mail viewer in the Mail preferences.

And try the fix in this support document: See the link below:

If Share options or Markup is missing after upgrading to OS X Yosemite or El Capitan - Apple Support

Yeah, this didn't work for me. No matter what I check or don't check, run or don't run in Terminal, the sharing options on Photos only show ICloud and Airdrop, neither of which I use.