and so on, until 10,000,000? Perhaps Facebook starts opening every link using 10,000 parallel threads. Can you really replicate that from your connection at home? Perhaps even the sysadmin of your victim site has whitelisted all Facebook IP addresses so their crawlers get a free ride.
There's a fair chance the service which generates these outbound requests throttles itself, both with respect to how many requests it makes against any one domain/IP, or how many errors it will provoke per time period, or before human review.