According to http://answers.google.com and http://howstuffworks.com, a commercial passenger jet with an average load gets about the same number of "miles per gallon per passenger" as a car with 2 or 3 passengers.
Interestingly enough, a small motorcycle with one person may get about the same number of miles per gallon per passenger- 60 or so.
if reducing CO2 emissions is your goal, riding in a car alone is pretty bad, but it depends how much you do it. Riding thousands of miles per year in airplanes and on motorcycles can also cause a lot of CO2 emissions.
To be honest, I take more airplane trips than most people. (i'm not sure how i compare to the average american, though.) My parents are in Alaska and I'm in Pennsylvania. I plan to visit them about once a year in the coming years but in the past few years have taken cross-continent airline trips twice a year.
So how much do all of you use airplanes, out of curiosity?