In my opinion, it totally depends on the bird, the situation, the household, the working schedules of the household, and a million other factors. I think some birds, if they get plenty of out-of-cage time and tons of attention from their owners, can be perfectly happy as single fids. However, these are FLOCK creatures we are talking about. I don't believe that they can be happy if they are alone all day long.
I have 14 birds total--whenever possible, I have them paired up. I've got a male sun conure living with a male lovie. A female greencheek living with a male lovie. Two budgies together. Two cockatiels together. Before anyone freaks out about the living arrangements--the odd couples have been together since they were 8 weeks old, and have lived happily together for over 3 years now. They feed each other, snuggle together at night, preen each other, etc. They also tend to hang out together when they are let out for their exercise time.
My larger birds--three CAGs and a goffins--are all rehomed, older birds. They all have their own spacious cages. They do not get along when let out of their cages. I have a large dog kennel (6 feet by 10 feet) that I've converted into an aviary. It's got perches (closet poles), toys, and a tarp roof. I can put the two female CAGs in there together on opposite sides. But eventually, Optimus will start to climb over to Rosie and Rosie will start to scream. I'm pretty sure Opti's just teasing. It's fun--something to do. Same thing if I put the male CAG in with Opti. They'll sit on opposite sides for a good, long time--playing with toys, trying to bite through the zip ties that hold down the tarp, calling out to passing dogs, etc. But eventually Loke will start to climb over toward Opti, and Opti will scream and head for me. Polly, the Goffins, doesn't interact nicely with any of my other birds, either. But they do communicate to each other pretty much all day long. They'll mimic each other's vocalizations. I'm pretty sure they keep each other company when I'm not home, even though they can't be trusted to share the same space without supervision.
As an aside, one of my all-time favorite things to do on a summer evening is to bring a few birds out to the aviary, plunk myself down in the canvas chair that I keep in there, and read a book. The birds'll wander over when they want a scritch, call out to passing walkers, whistle, watch the wild birds. Until one of the birds decides it's time to tease another, it's pure bliss.
