Possible Library bug - Unable to load the requested class

So, you have /libraries/facebook/Facebook.php which you're loading by calling $this->load->library('facebook/facebook')? This should work, as long as:
- your subdirectory (facebook) capitalization matches whatever you used in the call to $this->load->library()
- you used / in the call to $this->load->library() instead of \
- the file is named Facebook.php
- the class is named Facebook

If it's working locally, but not on the server, this is the bulk of the potential issues, as long as the error is coming from the loader attempting to load your library, rather than something in your library trying to load something in the Facebook SDK.

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide