Drag scales with air density. Up at very high altitiude - the air is much thinner

http://www.sablesys.com/baro-altitude.html

Now - Kittinger probably got close to, but not exceeded the speed of sound when he jumped in 1960 from 31.3km altitude - where air pressure is only about 16% that of the air pressure at sea level.

Whilst the actual maths of this are very complicated - it's easy to consider the typical skydivers terminal velocity at, say, 5000 feet is about 120mph - scale up to the much lower drag at higher altitude and you can get figures of over 600mph.

All you really need to do is jump higher - perhaps 40km - and a supersonic skydive is not beyond the realms of science - a team called Stratoquest intend to try in the not too distant future.