A problem with that is that people move, so zip codes change. You could fix that by substituting artificial, user-chosen groupings for zip codes if you used enough of them to have a similar distribution. Users could pick a grouping based on their interests, and you could even keep the geographical aspect of it by treating them as "neighborhoods". People who play the stock market could choose WallStreet, science fiction fans could choose Area51, wine aficionados could choose NapaValley, and so on.