Craigslist has 50 now, and had 28 as of 2009.
Reddit IIRC was 6 employees until they were spun out of Conde Nast in 2011. (Some of those early employees are on HN; maybe they can correct me.)
PlentyOfFish was one guy until 2008, 5 years after its founding. It was doing a couple million in revenue then.
The big labor-intensive function seems to be enterprise sales. If you're just building a product for people to use or charging self-serve for use, you can do that with a small engineering team. As soon as you start charging $100K for the product or ad sales on the product, then you need to start hiring people to convince customers why they should spend $100K for your product. Of course, each salesperson makes more than they cost, so it's still rational to hire them.