My guess would be that there were other reasons people objected, not least of which would be his nuclear follow cost. (The follow cost metric, the milliscoble, takes its name from him. That alone should tell you something.)
All snark aside, though, I think they most likely objected because the ethos of Facebook is completely different.
Facebook is mainly used for networking with people you already know and showing them what you have become or are becoming, whereas Twitter is largely used for sharing and developing ideas. Both social networking spaces are full of people sharing the latest news, but the idea of what's newsworthy greatly differs between the two. Status updates on Facebook are mostly about personal news and life events, whereas Twitter seems to have developed into a much more entrepreneurial, collaborative space.
I've certainly gotten the "magical experience" Scoble speaks of from Twitter. It just comes about in a different way, through sharing and building on ideas, interesting turns of phrase, and plays on words. Facebook isn't the sort of space where that sort of interaction can take place, in part because it doesn't foreground raw text the way Twitter does. Moreover, an account on Facebook automatically invokes a set of inescapable preconceptions about who you are, in the form of data re: where you come from, where you were educated, where you live now, who you work for, etc.
How did he manage to miss that fundamental difference between the two social networks?