[WT Sharpe]

I agree that an mdash should be used without spaces, but there are other conventions that seem to be falling by the wayside because of their incompatibility with word processors, chat-rooms, and readers. For example, I learned long ago that two spaces should follow the punctuation mark that ends a sentence. However, if you follow that convention today, many of your following sentences will begin on the next line after a space, which is awkward. For that reason, I usually use a space before and after the em dash. If it were in a printed book—by all means—there should be no spaces.

I also use a space after the em dash just before the given source in quotations; but in that case, it's only because I find it more aesthetically pleasing.