To be blunt, GameFAQs is losing users because games are becoming easier and easier to play. Nobody gets "stuck" in a game these days, because the games always have a difficulty setting that removes all the challenge from it.
Everything is becoming more spoon-fed. You don't really go 'hunting' for stuff in games anymore.
edit: There's also the fact that in-game guides are becoming more popular, and anyone will take a guide from a developer/well known player over a guide from a random nobody.
GameFAQs' guides can't really compare to stuff on playdota, smogon, teamliquid, etc.
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maybe, lol