The problem is that he's in Canada. The $800M is for violations of the CAN SPAM act, but Canada doesn't have any laws comparable to CAN SPAM. (Indeed, they have no laws against spam at all, other than the weakly enforced PIPEDA privacy law.) This means that should Facebook try to enforce the injunction in Quebec, he'd have a reasonable shot arguing that what he did wasn't against Canadian law, hence it's not collectible.

Facebook's original complaint also invoked Federal and state computer tampering laws, but the judgement doesn't refer to them. The judgement also enjoins Guerbuez from ever using Facebook again, so if he does, Facebook can go back to court, get him cited for contempt, and try to enforce that in Canada. They'd have a better chance doing that, but who knows how long it will take.

As has become increasingly apparent, the key to legal success against spammers is close international cooperation, since the various pieces of illegal spamming operations, botnet infections, botnet spamming, order collection, and making and delivering goods are often spread all over the world. You'd expect Canada and the US to work well together here, but for some reason, the coordination so far has been less than impressive.

Updated update: A friend who has extensive experience with Canadian law wrote to point out that the defendant is in Québec, where the rules are different from the rest of Canada. In particular, he notes that if a Québec Court finds it would not have jurisdiction over the action as brought by Facebook, if the matter was commenced in the Qué Courts, then the foreign judgment is not enforceable in Québec.

Since Facebook is in the U.S., and the judgement is mostly based on CAN SPAM which has no Canadian equivalent, it is not clear what a Canadian court would do. The situation would be a lot simpler if Canada had an anti-spam law.