As just a guess, I would say that the majority of people with in-ground pools use a pool service while a minority of people with above-ground pools use one. I think it mostly tracks with the cost of the pool. Those that can afford a more expensive pool (in-ground pools generally being more expensive than above-ground pools) can afford to pay for a pool service more readily. More older pools are manually dosed and more of those likely use pool services. Newer pools often have saltwater chlorine generators and while many still have pool services, more people probably take care of those pools themselves.

In my own neighborhood where there are mostly in-ground pools, it's a mix but with the majority using pool services. Not me, of course, unless you count me as the "pool man". I have a strong suspicion my wife is sleeping with the pool man.