How does Hitagi fall if she's weightless?
As @AkiTanaka mentioned and by checking Volume 1 of Bakemonogatari, we can see that she's not exactly weightless. She weighed 5 kilograms. To quote from the light novel:


  You’ve noticed, haven’t you?...That’s right─I don’t weigh a thing..Well, it’s not as if I don’t at all─a girl my height and body type would have an average body weight in the high forties, in kilograms...But my actual body weight is five kilograms...Now, if you want to be completely accurate, it’s only that a scale displays my weight as five kilos─I personally don’t notice it....


Her saying she weighs nothing is just an exaggeration or a figure of speech. She doesn't really weigh nothing, it's more like a hyperbole, since the difference between 5kg and an average person's weight is noticeable.