I've also been the only passenger in F once. It was probably 8 years or so back and I was flying JFK to BOS (due to flight canceallations my original LAS-LAX-BOS, with no redeye had become LAS-LAX-JFK-BOS with a redeye). Very short flight and couldn't ever see myself upgrading a JFK-BOS only flight.
A few months ago, DEN-SAT , was one of 2 in F on an A319. Several years ago, was in F on ORD-NRT in the old extra large F and C cabin (was that the 747 OP?, it had like 40 F seats), there were 3 of us. We were all so scattered over that huge cabin, it seemed like it was my own! (That was also back in the days of the allowable 'double upgrade'...Ahhhh, the memories!)
In most cases, I'm just elated when I have an empty seat next to me. More than 1 or 2 open F seats is rare these days.