what are the best out-of-the-box solutions for a commenting system?

I'm looking for a solution to build a commenting system like Facebook has it with nested replies. I prefer a service to easily put things together, such as sendbird, though sendbird doesn't seem to support that usecase. Are there any other popular solutions out there specificity focused on chat / comments to get it done?

Note I'm looking for an SDK, not something like Disqus since I want to hook into the chat and customize the UI