They are therefore trying to brute force the first 11 characters of the address. The author of scallion estimates one can achieve 520 MH/s with a AMD Radeon HD5770 GPU [2] which retails for $190 [3], they then give the formula for calculating the time (in seconds) to have a 50% chance of finding a matching URL: 2^(5length-1) / hashspeed Which with a length of 11 and a hashspeed of 520M, would take about a year with the one GPU [4]. The total cost of the hardware to have a 50% chance of finding the vanity address "facebookwww?.onion" within a week would therefore be around $11 000 [5].
Imho, this is well within the realms of possibility for a company as large as Facebook and does not suggest a weakness in the .onion scheme.
[1] https://github.com/lachesis/scallion
[2] https://github.com/lachesis/scallion#speed--performance
[3] https://www.amazon.com/dp/B0032F63TW
[4] http://www.wolframalpha.com/input/?i=%282^%285*11-1%29+%2F+5...
[5] http://www.wolframalpha.com/input/?i=%24190+*+%282^%285*11-1...
[edit]
Dustcore is very correct, correcting for my initial mistake, it would take 1.1 million years on a single GPU using scallion. Finding that sort of result in a month would require $2.6 billion worth of GPUs. Now I know facebook is known for spending billions on questionable purchases, but this would be a bit extreme even for them. How the hell have they managed this?