I've not reached this area as of yet so its quite new to me about the comments part as I had no idea there was even 3rd party ways of doing this.
I built a social networking site on WordPress, his is how it led to me becoming a full stack developer but this is one of things towards the end of the project made me question. At the time I was on a shared simple hosting package and when I wanted to upgrade for more traffic just in case.. that's when it starts costing.
If you are unsure what it might cost you depending on the traffic I would suggest going with NGINX and AWS, Google servers which offer these scale-able solutions. However it can get expensive I would imagine if you're getting as many comments like Facebook.
I would ask the owners of Hashnode as they might be able to point you in the right direction :)
After a quick google, this gave me a bit more clarity - hongkiat.com/blog/3rdparty-comment-discuss-system…
I do recall someone writing about a similar product called Disqus
Hope it helps and thanks for the invite! :)