Incredibly, PIK-STN peaked at over 70,000 passengers per month in late 2003. Since then there has been a steady decline in passenger numbers as you can see from this time series plot of CAA data:
However, if you look at the wider Glasgow - London market you can see that PIK-STN is not the only route having problems. The market has declined by 24% since its peak in 2003 and is now at its lowest level since the late 1990s. Almost every route between Glasgow and London has suffered, including GLA-STN, which peaked in 2006 when Air Berlin flew alongside Easyjet:
