I hear these types of questions about many web companies from time to time in that the assumptions is you just need a small bundle of folks to run the website but in reality when a business gains traction the website is just the tip of the iceberg in many cases. The number of employees needed can snowball as the business starts to take off... legal teams, finance, accounting, QA, HR, BAs, management, ops, etc - it all adds up to usually more than one would assume. Additionally the customer facing product can be a very small part of the overall tech solution in that developing needed internal tools and systems can require large numbers of employees.

Typically your biggest numbers are going to come from sales folks / account managers in that, as an example, if the business model allows for a sales rep to sustainably bring in 200k on an 100k salary you most likely are going to hire as many as you can get. Groupon is a good example of this in action although it obviously has not been as successful as they would like.