What is the optimal way to manage copyright on our online platform?

Your platform's terms of service should contain provisions acquiring copyright interests in user submissions. Material "you" create is owned by "you," but this is presuming 'you" are some entity that acquires rights to its material through IP assignment provisions either buried in operational documents or agreements with ICs. Materials your employees create in the scope of their employment are presumably works-made-for-hire under the Copyright Act, but you would want provisions in place acquiring a broader scope of rights to their work.

Acquiring rights to materials produced for you by third parties would also occur through provisions in an agreement (e.g., blogger submission agreement or independent contractor agreement). However, paying bloggers brings up Federal Trade Commission issues and requires that certain disclosures be made.

Speak with a local IP attorney to review your specific content strategy and obtain appropriate IP rights-acquiring instruments.