That's a good point. Nitrogen fixation is about 3e8 tonnes (3e11 kg) per year http://nmsp.cals.cornell.edu/publications/factsheets/factshe... so the 4e18 kg of nitrogen in the 5e18 kg atmosphere have a half-life of only about 1.3e7 years. Loss to space is not significant for heavy gases like nitrogen. So you are breathing mostly the same nitrogen molecules as the person who first started a fire or flaked stone into a hand axe, but mostly not the same nitrogen molecules as dinosaurs.

A thing I'd like to know is how big the non-atmospheric reservoirs of nitrogen are. When nitrogen is "fixed" out of the atmosphere and into an ocean or a pile of bat guano, how long before it cycles back into the atmosphere, on average? A hundred years? A hundred million years? I'm pretty sure it's in that range because nitrate rocks are rare but used to support most of global agriculture, but I don't have a good idea of where it is in that range.