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By far the most common consulting question we get at Mashable, at least twice a day, is which platform or white label solution is best to base your social site on. There's not one single answer, but in quite a few cases we suggest Drupal, a free, open-source platform for community-driven sites. It powers The Onion, Spread Firefox and a number of sites reviewed here in the past - ProjectOpus, BuddyPing, Placeblogger, Hypercomics and more.