"Supply is far outstripping demand. Despite our city's ever-growing obsession with green commuting, interest in carpooling appears to have plummeted. At the end of 2008, 352 car pools and van pools had registered to park in the city's reserved spaces. That number has since shrunk by more than half, to 140.
Today, only 88 car pools have permits from the city of Seattle. Yet there are nearly three times that many parking spaces in those Sixth Avenue lots alone."
This article from the Seattle Weekly suggests that the downturn in carpools in Seattle has to do with the city's move in late 2008 to double the cost of carpool parking permits to nearly $600 per quarter.