- They may not be counting support staff/abuse monitoring staff as employees (they might be contractors). Even if they do, it's possible that 10-15 people could do the job, depending on their support volume load.
- Probably very little management overhead. This also increases productivity across the whole company, as there is a lot less communication overhead. Let's give them 4 management types, including Craig, Jim, a CFO and an engineering manager (if that's even necessary).
- They don't do much feature development, so they might only have a few engineers. Let's be generous and say 5.
- Running their network operations probably takes quite a few people, but with a relatively simple website, it can definitely be done with a small, efficient team. Let's say 3-5.
- Let's throw in one office manager/executive administrator, to make everybody's lives easier.
That totals 30, even with support, but there's still some wiggle room. If support staff or abuse monitoring staff are contractors, there's more than enough room to work with.
With the right number of engineers and system administrators, there is no need for a higher overhead when you already have a well oiled machine. Throwing man power at something doesn't make things more efficient. You just need the right talent and you can do bigger things with less people. [2]
So the question for you is, why would you expect or need more?
[1] http://www.quora.com/Craigslist/Why-has-Craigslist-innovated...
[2] http://radar.oreilly.com/archives/2006/04/database-war-stori...
I don't think you need more. But why do many other companies have more?