Trademark law affects brand name usernames. If someone registers @nike on your platform and uses it to sell counterfeit goods, both they and potentially you face liability. Most platforms solve this with proactive reservation plus responsive takedown processes.
Impersonation has criminal implications in many jurisdictions. Pretending to be a government official, medical professional, or financial institution can violate fraud statutes. Your username policy should explicitly prohibit impersonation with professional consequences.
Sector-specific rules add complexity. Fintech platforms may have KYC requirements that affect username verification. Healthcare apps might need to verify medical credentials before allowing "doctor" handles. Gaming platforms have age-related compliance considerations.
Safe harbor provisions protect platforms that act in good faith. Document your reservation policies, respond promptly to complaints, and show consistent enforcement. Courts are more forgiving of platforms that demonstrate reasonable effort.