While i like the sibling comment's network errors answer better then mine I didn't see any reasoning other then a unfounded statement[1] in the article or on a quick skim for "typo" in the referenced whitepaper:
[1] "These requests were not typos or other manually entered URLs"
Here's a better quote:
"There are many frequently resolved domains that few people type or know."
There's a lot more details in the article.
"All of these requests used only four domains in
the HTTP Host header, as shown in Table 4.
Three out of four domains contain more than one bit error, ruling out a simple
mistype of fbbdn.net for fbcdn.net. "
Not to mention, the number of people typing "fbcdn.net" into a browser has to be vanishingly small to begin with.