I am writing this post as an experience that I had faced while attempting to upload a large number of pictures from Photos in OS X El Capitan. The share feature of Photos only allows the sharing of ~30 pictures at any one time. As such, it can be a pain if you have hundreds of pictures that you wish to upload.
A below is a good workaround:
1) Export all the pictures via File > Export > Export X photos.
2) Check include “Title, Keywords, Description, Location” if you want your upload on facebook to contains those details as well. Simply fill in the description for your photos before exporting.
3) Choose Sequential under File Name if you want your upload to be in sequence.
4) Choose a destination folder and click Export.
5) Upload to facebook on the web.
While the above is not exactly the best solution (thanks to Apple Forum), but it works.
We hope that Apple will allows the uploading of more pictures in future release of Photos.