I don't think it takes billions to build a platform but it does seem that it usually requires millions to get more than a few people using it.

I have accounts on loops, lemmy/piefed, mastodon, element and more. I've never met a person IRL that uses them and I've never gotten to know anyone in an online community (gaming, interest forums, etc.) that use them. Viewing my loops app makes me feel like I'm alone on an island. The communities I follow on Lemmy/Piefed will go weeks without a post. The communities I manage will go the same without a submission from the community. Now that my immediate family have grown tired of the bugs on element after I forced them to use it to chat with me, I am literally the only person I know on the messaging platform.

I love open source but if I were the type of person that required a true sense of a well-rounded social community, I would use something like tiktok to scratch that itch.