Earlier quoted context omitted.
According to the article, facebook has a .onion domain.
The article says its https://www.facebookcorewwwi.onion
The process is described in "Part three" here: https://blog.torproject.org/blog/facebook-hidden-services-an...
"The short answer is that for the first half of it ('facebook'), which is only 40 bits, they generated keys over and over until they got some keys whose first 40 bits of the hash matched the string they wanted."
"Then they had some keys whose name started with 'facebook', and they looked at the second half of each of them to pick out the ones with pronouncable and thus memorable syllables. The 'corewwwi' one looked best to them — meaning they could come up with a story about why that's a reasonable name for Facebook to use — so they went with it."
(Corrected: Hash of public key not private key per itsbenweeks below)