It depends how many millions of messages were sent, and how much % of those converted to a sale, and how much % each sale generated, and how many different direct sale programs are involved. In the e-mail spam world, yes spammers will generate 7-digit earnings.
But $873 million is an awful lot, even for a very prolific and successful spammer. And facebook is - compared to most networks - extremely clean. I've never received a single spam message on facebook, other than those hourly status messages from my cousin (The internet will regret the day she discovers Twitter)
This is a landmark case for F8, they are very clearly saying "we will catch you and we will destroy you". Good for them!
I'm trying to imagine how the perp pulled it off, and the LA Times article doesn't divulge much. Was it all conducted within the facebook network (ie, facebook messages, notifications, commentspam, etc), or was the spam delivered outside the network (email)? Stealing people's facebook passwords is enough of a phishing challenge as it is.
Now ... is that $873M in Canadian dollars?