I didn't fly last week but the lightest flights I've had:
RIC-BOS on US Express ERJ - 6 passengers
BOS-LGA on US Shuttle A319 - 20 people
BRU-LHR on BD A320 - Approx 20 people
LHR-IAD on BA 747 - 4 pax upstairs....who cares about the rest of the aircraft?

EGDM-EGDM on Grob Tutor 115e - only one person aboard...me!
There are several threads in Travelbuzz (well in the archives) that refer to flights with light passenger loads.