Instagram was also extremely lean (13 employees, maybe?) from what I recall.
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.
In the case of at least the last 2 they also used community moderators which can be seen as a tremendous HR hack in some ways
A lot of times enterprise sales folks are not particularly technical, so they'll often request support from a sales engineering organization.
Consider if you wanted to build a Spotify competitor. Disregarding the partner management/sales/distribution side of thing, but sticking solely to the tech:
You have to be able to ingest content from all your partners. If a new song is supposed to go live exactly at midnight on a given day, you gotta be able to deliver that. Not just media, but also systems for ingesting, reviewing, and publishing the metadata, album art, etc.
You have to run on a lot of devices. You need a desktop or web app, an iOS app, an Android app, a bunch of living room apps, Chromecast support...
You have to be able to bill people. I'd suggest offloading as much of this as possible so you're not touching credit cards yourself, but then it's still integration work. And now you have paying customers so you're going to be expected to have good reliability and responsiveness to issues, and to build tooling for your customer support team.
Paying customers and inventory - even digital inventory - make things turn into a lot more work quickly.
There is almost no software priced between $1,000 and $50,000. Only big companies can afford to pay $50k+ for software and as a result they want a lot of hand-holding, support, and customization. Because they're spending so much they also want a lot of CYA assurances. Let's also be honest: they also want sales people to buy them lunches and send them bottles of scotch.
Selling to enterprises for $15,000 is a great way to go out of business: you won't be able to pay for enough staff to keep the sales pipeline flowing and you're too expensive for individuals and small businesses no matter how much value you deliver.
I wasn't that early, but I was probably in the first 30 employees, depending exactly how you count things.
There are start dates for most of the early employees in this file that used to power the (now removed) "team" page on the site: https://raw.githubusercontent.com/reddit/reddit-plugin-about...
The "new" field in that file is YYYYMM data for when each employee started (though, looking through it right now, some of these definitely aren't correct, but they're reasonably close at least). Figuring out when people left is a little trickier, but estimates around a few significant dates:
* Acquired by Conde Nast (Oct 31, 2006) - 4 employees
* Independence from Conde (Sept 6, 2011) - 12 employees (including 2 redditgifts from that acquisition the month before)
* $50M funding round announced (Sept 30, 2014) - 58 employees (many working on redditgifts/redditmade)
* Steve Huffman returns as CEO (July 10, 2015) - 69 employees
* $200M funding round announced (July 31, 2017) - 230 employees
And they've said that they're aiming to get to 300 by the end of 2017.
Instagram was also not profitable yet. They would have easily ballooned to 100 people if they'd tried to become profitable.