Bret Terrill, CEO of Tenuki, identified 4 reasons in a recent blog post: 1.You’re really you on Facebook. In most situations where avatars are successful, users are masking their identities… 2.Avatar applications don’t encourage repeat usage. I customize my avatar once and I’m done… 3.It’s all lead-gen. The goal of most of the well-done avatar apps is to drive you from Facebook to another property. That’s not the way to grow a Facebook app. 4.No virality. None of the existing avatar applications have any viral hooks build into them, except the common “Here’s 50 coins if you add a friend” invite page… - Graham Sergeant