I'm guessing you actually mean it covers running costs on a small amount of ads. A small amount of ads doesn't pay many wage bills* - I'm guessing there are a equivalent of a couple of full-time people working on SO? Doing backups fighting off crackers, ensuring trust metrics aren't abused, making optimisation changes, upgrading hardware, etc..

500000 uniques but how many ad clicks? I'd imagine their clicks-per-impression is really low on this sort of site unless the ad is the solution to the problem. People are focussed not aimlessly searching, they're also probably the sector most immunue to ads - computer experts at work.