How many employees do we need?

My co-founder and I are in the very early stages of launching our website. My day job is in finance, a highly-intense industry, to say the least. I very rarely have downtime during the day and our company is run very efficiently. If our website is ever fortunate enough to get big I could imagine my co-founder and I hiring a "few" employees...enough to support new features, api's, deal w/ customers, etc, yet lean enough to keep the employees busy during the day.

What I don't understand is the amount of employees some of these startups and tech companies have: Facebook w/ 2,000 or so; Twitter w/ like 600. I mean, their websites are already up and running. I understand that you need a few people to maintain API's, deal w/ advertisers, etc. But 2,000? 600? Really?

Am I just under-estimating how many people we could eventually need? Websites are supposed to scale, right? Why do these guys need so many employees? They must be stepping all over each other, right? Do they just have a crazy amount of downtime? Maybe it's a competitive issue....facebook/twitter hires a bunch of people in order to deter other companies from trying to compete.

Spend some time on LinkedIn searching for Facebook employees, and you'll start to get a grasp of the wide spectrum of jobs that Facebook does on a daily basis. Serving 800M users and generating more than $1B in revenue is no small feat. They have all the usual departments, from R&D to sales and marketing.

How much customer support do you need? How many third-party app developers are there? What happens when they ask a question? Who organizes the Facebook developer conference? Who handles the 1,000 media requests Facebook gets on a daily basis?

The list goes on and on.

Now, in regards to your startup: don't worry about it; you'll staff your company as the needs arise, one employee at a time.