I was going to comment that Google, Facebook and Twitter are platforms for search, connections and communicating respectively. Ads are simply a revenue model. If Facebook charged $20 per month, would you work for them then?

However, after seeing comments like: I work for facebook - "advertising is itself a worthwhile problem."

and... I work for google - "People in non-advertising teams hardly ever ask 'how will this effect click-through rates?'."

Hardly ever? :-) Maybe it is more about advertising then I realize.