I got the same ones and they work very well.
Polypropylene is used because it is an inexpensive polymer that is melt spun into filaments which have appropriate mechanical properties (strength, modulus, bending), but other (non-water soluble) polymers would work well (don't know if other polymers are used for hairnets; poly(lactic acid) or other polyesters would be better as they are biodegradable but much more expensive than polypropylene or polyethylene). Spun-bonded refers to a thermal after-treatment of the polymer fibers directly after continuous filament melt spinning of the non-woven fiber web/mat, which causes some fiber junctions to be melted together. The bonding of the junctions provides additional strength and dimensional stability to the non-woven fabric, and also allows the fabric to be much thinner and breathable/cool. Traditionally fibers were woven or knitted, but the more recent random orientation fiber mat which is much cheaper to produce and more versatile for many applications is referred to as a non-woven (since most fibers are woven rather than knitted). A non-bonded non-woven would likely disintegrate in the skimmer basket which would send filaments/fibers to the pool filter.
I believe normal skimmer socks are ring spun (knitted) cellulose/cotton and thick both for reuse and to reduce porosity inherent in knitted fabrics. Hairnets may have an environmental impact, but I would be surprised if the polypropylene used in each one (i.e. minus the elastic) is more than 0.1g.
