F.A.Q: General Guidelines.

Ticket #00003 : Facebook SDK Lexical Preprocessor Error

1. Delete the FacebookSDK framework from your project.
2. Delete any references to the Facebook SDK from your “Framework Search Paths” and “Header Search Paths” in Build Settings (be aware this setting exists both under ‘Project’ and ‘Target’, so clear it from both places)
3. Clean your project, save and quit.
4. Relaunch your project.
5. Re-add the Facebook SDK framework by dragging into your project.
6.) Check the two checkbox.
7. Compile. At this point, it was fixed for me.

Credits to:
dazzer37