Personally I know the numbers of the (at a guess) 20 people I call most often, but I always assumed that, in 2011, I'm the exception not the rule.
Not particularly relevant, his point stands either way, just interested to know if other people on HN still find themselves memorising phone numbers.
So far, 100% success rate.
Like Jebdm, the only numbers I really know are those that haven't changed in a few decades and my own cell number. But I also prefer face to face conversation over telephone, so I'm only likely to call you if I can't find you. (And then I'm more likely to just text or e-mail....)
At least it was until my cellphone stopped playing back DTMFs for stored numbers. SIP and quiet dialing have almost killed the music.
I can also tell you my grandmother's number, but not her address. Again, I'll visit her maybe twice as often as I'll call her (not that many times in a year, she lives 60 miles ago).