Digg and Reddit, for starters, don't have hundreds of their own datacenters - and about one or more orders of magnitude less users than the "giants". Twitter seems to have operated a mixture of leased and owned DC space (http://www.datacenterknowledge.com/archives/2011/09/19/twitt...), but far, far less than FB/Google/AWS do.

Digg and reddit are fairly niche, at least for advertisers - which means these sites need far less ad salespeople and support staff; reddit also can get by with having next to zero visible support staff by relying on community moderators which cost them nothing.