I have been doing AdSense advertising for quite awhile on numerous sites so let me see if I can help people who are just starting out.
The key to ad revenue is the topic your site is about. I have sites about peace that need a lot of traffic and clicks just to make a respectable monthly amount. On my peace sites, each ad click usually gets about 10 - 25 cents per click. However, I have a few investment sites and the ad revenue multiplys much quicker. I usually average about a dollar per click on the investment sites. So the first thing to consider is what your subject area is and how much each click will be worth.
The second thing to consider is how you are getting your traffic. If you are paying to get traffic (advertising on other sites, radio ads, search marketing, etc...) then you will need to figure out how much ad revenue you can make to cover these costs. If your traffic is coming in for free, then it will be much easier to make money.
You also need to be aware of click through rates. You need to test your ad performance to see what works in terms of getting people to click on the ads. Even if you have an investment site with good high paying ads, you won't make any oney if nobody clicks on your ads. You need to test, test, and then test some more. Try different ad placements and different ad colors until you are happy with your clickthrough rate. One small change in your ad location or color can double your click through rate and that will double your income.
Testing all the possible changes can be time consuming, but it is worth it. Once you find somethig that works on your site, it seems to continue working even though new people are coming to your site each day. Their eye flow seems to be the same and the click through rates seem to be the same for new users.
On social networking sites, you also need to consider that new users might click on ads more than returning users. Returning users tend to get "ad blindness" as they return for the content and learn where the ads are and begin to avoid looking at them in order to not waste time in their search for new content.