According to the book and Wikipedia, there was even a time when Mark was even more focused on another idea of his, Wirehog. It's possible he may have even had some doubts about Facebook, or lost interest in it for a little while.
And hey, what startup founder hasn't had those thoughts? They're totally natural. I'm sure some have a razor-sharp focus & vision from Day One, but I would never fault a founder for having a few shaky moments of doubt here & there.