How do paper wasp larvae not fall from their nest?
Social wasp larvae hatch from an overhanging egg, and they grow up through a series of skin moults, typically 5 in number. The shed skins accumulate terminally behind their pointed anus, where the egg chorion peduncule remain. These wasps have a special body construction and skin texture (e.g. abdominal lobes, numerous spikes) that allows for pressing against the cell walls.   

You can see these traits from high-resolution images shown in the larval descriptions:
Florida Entomologist, 95(4):890-899 (2012) https://doi.org/10.1653/024.095.0411
Sociobiology 63(3): 998-1004 (2016) http://dx.doi.org/10.13102/sociobiology.v63i3.988

Finally, it should be minded that gravity doesn't affect small insects in the same way it affect larger mammals like humans. This means it way takes less effort and sticky secretions for these bugs to hang onto walls than we typically imagine by common sense.

I don't know of any dedicated estimations on fallen larvae, but I guess the event isn't common, unless perhaps in abnormally structurally-challenged nests.