The Queens Blvd ridership is low, because you have three additional trains running on the line. Seats on E and F express trains during the morning rush are pretty much gone when the trains leave Jamaica Center / 179th St. A lot of ridership on Queens Blvd trains originates from bus transfers. That is why the QB local trains, which begin in Forest Hills, do not get much ridership when compared to the expresses. The M-train providing 6th Avenue service directly from the QB local prevents a lot of transfers at Queens Plaza compared to when the G ran to Forest Hills.
The Brooklyn M is amusing. I love watching riders transfer at Delancy / Essex St. Riders will come in over the bridge on a Nassau St train, and get off at Essex St. There will be a chain of straphangers positioned at various points along the stairs from the upper level to the lower level where the F stops. And you'll see people wait to see which train comes first... The F or the M, and then you'll see the swarm of people move to that platform to get on the 6th Ave train.
The M is usually a shorter train (eight 60 ft cars), and has great ridership. I recall reading that a lot of gentrifiers were ecstatic when they were given a 1-seat ride to midtown with the re-opening of the Chrystie St connection from the Broadway (Brooklyn) subway.