Drupal - How do I enable comments for user profiles?

You can use Profile2, that attach node to user profile, so it can be commented and will look as commenting to user.

I have not tried it, but the Reply module looks really good.

Reply is a D7 answer to entity comments. It is not intended to replace core comment module, more likely it supplements it's inability to work with entities other than node.

Reply is basically a fieldable entity with subject and body fields which are hardcoded into module and are not Field API fields. This is because it is the essence of reply/comment. The entity is fieldable so you can create a 'review' or whatever you like by attaching fields to it's bundles.

Currently in Drupal 7 core, the comment module is still coupled to node.module. Comments are entities but can't be added to entities other than nodes.

You can see this issue to get the update: Decouple comment.module from node