I'm thinking that the main reason why the Halo 3 playlists died down is because of the fact that Halo 4 came out. As soon as people get tired of Halo 4, the playlists will make a comeback. We all hope, anyway.
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I've found games easily the last couple of days in the social playlists, and I know you can get games in high level snipers, slayer and doubles.