That's strange. I've been playing the PSN version several days a week for the past two weeks, and there was only one time when I couldn't find any teammates. I've even done about 6-7 Survival Mode sessions. To be fair, I usually play during the day, and usually most of the other players are in Asia. But there seems to be a lot of life in the online community yet. Try playing on weekend evenings for the best results for USA players.