Doing less than 80 in the carpool lane is a good way to get yourself killed/run over.
I would not recommend tailgating - especially in the carpool lane and especially in SoCal. There's a lot of road debris/trash/crap that gets pushed up against the center divider and often it can migrate over into the carpool lane. Your best defense against hitting something and/or losing control/damaging your vehicle is leaving enough room to see and avoid junk once it becomes visible when the vehicle in front of you goes past it.
I'm sure the Prius guy was just out feeling his smug anyway.
