Q: Cant post to Facebook from Photos. There used to be a simple way?

Is Facebook missing in the Share menu?

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

Open the link and copy and paste  the Terminal command exactly as it is into the Terminal window, don't try to type it yourself.

It is a harmless command, but spelling errors in the Terminal can be dangerous.

This command does the following: It will reset the launch services database, so the installed applications and the services, document types, etc. supported by the applications can be found.

Paths are searched for applications to register with the Launch Service database.