The ad revenue is probably insignificant unless you're a major faucet (like LandOfBitcoin.com or Dailybitcoins.org).  Typically, most ads are based on unique IPs.  Since the same ad is displaying across many web sties (most faucets use the same few ads), I think only the first one per whatever given time period is paid for the ad.

Mostly, for my one, it's about giving back to the community and potentially growing it into more a resource website.

I used a lot of faucets when I first started out and they helped me make my first few purchases with BTC.