The baked-in vs. bolt-on social issue has been explored by Jason Cohen, and I agree with him that baked-in virality is more effective than bolt-on virality. How

The baked-in vs. bolt-on social issue has been explored by Jason Cohen, and I agree with him that baked-in virality is more effective than bolt-on virality. However, consider also that Dropbox's referral program is a bolt-on social feature.