The OS X Photos program has a built-in Share button that can post selected photos directly to Facebook. However it has the following limitations:
Therefore I recommend the following procedure to upload photos from OS X Photos to Facebook:
In OS X Photos, select all photos that you want to upload by holding Command and clicking each photo.
From the menubar, choose: File > Export > Export N photos…
In the Facebook web interface, choose “Create Album”, selecting all the photo files that you exported in the previous step.