I just know a couple of ex-staff and one that's still there.
They are careful not to oversubscribe server resources though and do have a really solid development team that actually care about performance. I can't imagine a time where any customer could complain about speed issues and it actually being due to resource oversubscription.
I've sold/supported servers for large hosting providers in the past and my general recommendation for them was to never have more than 1000 per server as it makes for a lot of people affected with very busy phone lines all round when something inevitably goes wrong.
Nothing as bad as a popular competitor that have 6000+ accounts per server though and do just shift those who notice it around as
@visibleman mentions.