Generating traffic?

I see a ton of advertisements for traffic generation and I'm curious what the most effective way is..

I mean I could imagine owning a botnet would prolly be the absolute best way to generate traffic, but I see some are saying they can get it from Twitter, Flickr, MySpace, etc. How can they quantify something like that? Ensuring 10,000 visits or clicks when 10,000 different people getting the ad will not all click, and sending anymore could potentially result in a ton more or a ton less.

Any insight?

twitter will generate you 10.000 clicks but only about 800-1000 visits