This is one of the best blog posts I've ever read. It's Marco's recount of how Tumblr started. He was the first employee and built most of the infrastructure. Read the whole thing. This is how dreams evolve and become reality and then become world changing.
You can tell there is a lot of love between Karp and Marco. This is one of my favorite segments.
The One-Person Product – Marco.org:
"Even though Tumblr was never a one-person company, it usually felt like a one-person product.
David always had a vision for where he wanted to go next. I was never the “idea guy” — in addition to my coding and back-end duties, I often served as an idea editor. David would come in with a grand new feature idea, and I’d tell him which parts were infeasible or impossible, which tricky conditions and edge cases we’d need to consider, and which other little niceties and implementation details we should add. But the ideas were usually David’s, and the product roadmap was always David’s."