Getting paid for it could be a reason. After all GameFAQs message boards are pretty popular and it could very well be worth it to try to affect the attitude and discussion. If I was a console manufacturer and noticed my product being bashed on a somewhat popular message board focused on games and game consoles I would definitely think what I could do about it. I could try to either change the tone to a more positive one or try to destroy the credibility/usefulness of the discussion by derailing and trolling it. While there might be no way to tell if someone is a shill or not it might be useful to keep in mind that someone's stance might be affected by factors outside of the argument.
I see there seems to be some artificial "fanboy war" setup here. Some want to give the impression that people randomly picked either MS or Sony to support for some esoteric reason and now try to spread hollow propaganda just for the lols. I don't see why this would be true and it would seem much more likely that people at least try to choose rationally. For example if it was announced that the PS4 could only run games at 15fps I can see a lot of people changing to the Xbox One instead of staying.