I'm wondering about this aswell, i have a site i'm promoting on twitter, lets say 100-200 messages to targetted audience (its not a money site tho, its about dancing).
about 1/3 of those 200 messages comes and visits the site.. 1 or 2 of those might click an add... organic traffic at the moment is like 10 UV/day ... so twitter vs organic is 70/10 ...