I can only speak regarding UA, but I was alone in F BRU-IAD last summer. From my gallant perch in the front cabin, the flight looked to be pretty full in J and Y.

Better yet, I was non-revving as an intern. It stunned every FA on the aircraft.

(Perhaps fittingly, another flight back to IAD cancelled and its passengers were rebooked on the UA flight on which I intended to return, so I spent two days waiting to fly Y on another airline to get home.)