HIre writers and or friends and create tons of posts with tons of comments ... true or not. Though nowadays you could get something like Chat GPT to create all the fake posts and comments until you marketed where a few users come and interact with the AI content then a few more and more and more and then less AI fake crap and more authenticity.
Im pretty sure this is how Reddit and many popular sites started. Fake it before you make it! Without interesting content you DOA.