A New Yorker using the alias Phastman hijacked the computers of more than 77,000 Facebook users and used them to send junk emails to their friends.
Eric Crocker was one of 70 people arrested - 12 in the US - as part of the worldwide Operation Shrouded Horizon, in relation to cybercrimes organised on the Darkode hacking forum.
Crocker and others using the Facebook Spreader tool received $300 (£191) for every 10,000 computers they took over.
He will be sentenced on 23 November.
Crocker faces up to three years in prison, a $250,000 fine or both.
He pleaded guilty to violating anti-spam laws and the abuse of internet connections.