besides, they compare most favorably with greyhound or amtrak as intercity transportation, rather than intracity like SEPTA is. The SFO airporter I know nothing about, but it sounds like you're comparing another specialty service to SEPTA rather than a true mass transit service. When you think of private mass transit, you have to think of a private company that actively provides general transportation around an area -- the PTC was one, Red Arrow was another, Krap(f)'s is one, and there are probably a few scattered around the country that operate similar systems to SEPTA.
Basically, the fact that a private company moves people doesn't make it a good example of privatized mass transit (heck, some local private schools woudl qualify in that case)... mass transit moves the masses, not just the tourists (Phlash) or wealthy (taxis, assuming you take it on a regular basis).