I know of two: Craigslist and Plenty Of Fish.
Possible? Yes. Likely? Is the end goal to own a unicorn? Become a billionaire? Or something else? What are you optimizing for?
I am rich and have no idea what to do - https://news.ycombinator.com/item?id=42579873 - Jan 2025
This means that you will be dealing with 20M humans and 400k paying ones. They come with a lot of requests, questions and demands. From many jurisdictions. Not to forget tax laws.
At some point you will find it easier and cheaper to integrate rather than buy every service. And maybe even more fun to work with a great team.
assuming one-person means one employee, i think it's possible if you allow contractors. however, even then i think it would probably be more economical to hire employees. there's just so much diversity of skill needed to maintain and monitor a service at that scale. of course it depends on what the business is. maybe there is a niche that works.
the way you present the scenario isn't conducive to achieving "one-person unicorn" though. the way to do it would be for an already successful entrepreneur to raise a lot of VC money at a $1B valuation before having a product or customers at all.