Unlimited realtime database refreshes times millions of PSN users = network overload (but not an overload of your PS4). I would greatly prefer the infrastructure to be geared towards robust gaming, not managing friends lists over 2000 people for PSN users that can't be all that selective. Think about it: 2000 is a lot of people.
It helps to remember that every last option we are afforded on PSN is afforded to all connected users at any given time.
If Facebook can handle it, I'm pretty sure the PSN should also.