For comparison, Facebook had 3500 employees at the time of their IPO, and it's "just a website." Snapchat is many products: video ad platform, augmented reality selfie app, broadcast platform via sunglasses, photo uploading app, and messaging app.
whatsapp had what Facebook had ÷ 100.
I'd imagine that someone like Facebook, Twitter, Snapchat, etc, who makes the product at available $0 and has to devise a monetization scheme later will need to employ a fair amount of "soft skills" staff who can forge relationships with, say, advertisers, and gobs of staff to work on features, stability, capacity to achieve a point where their platforms attract and retain eyeballs. These are the sort of companies where an injection of VC money to rapidly scale is beneficial such that the product can achieve monetizable critical mass, to fill out the part between "2. ???" and "3. PROFIT!"
[1] http://www.forbes.com/sites/parmyolson/2014/02/19/exclusive-...
[2] without paywall: https://www.quora.com/How-was-WhatsApp-built
And by their own admission, they intentionally tried to slow user growth.