Comment threads are not “social media”, no matter how badly anyone wants to redefine that they are. They date back to the 1970s on USENET and mailing lists.
Because the phrase 'social media' was coined to describe a very specific thing: people who know each other in real life communicating online about things happening to them in real life. There were and are plenty of other words you can use to accurately describe other things, like 'chat room', 'forum', or 'link aggregator'.