The question is basically saying that Facebook started with this onion address, then set out to generate the corresponding private key. This premise is incorrect, as pointed out by Steve’s answer: in fact, this task would be infeasible using current hardware.
Where could this incorrect premise have come from? According to Facebook’s announcement , their onion address can be read as “Facebook’s Core WWW Infrastructure”. The fact that the onion address has such an elegant meaning might suggest that they must have generated it on purpose. Humans are surprisingly good at assigning meaning to randomness.
The real origin of this onion address is described in a post on the tor-talk mailing list by Alec Muffett, as cited in Steve’s answer: There are eight custom characters, and they got lucky with the rest. There is a later, more detailed post in the same thread by Roger Dingledine, which suggests the Wikipedia article “Birthday attack” as reading material. These posts were cited in a ZDNet article about Facebook’s onion service .
 Of course, this is also available via an onion service.
 This article incorrectly suggests that Dingledine’s post came first.