Similar to this: before cellphones stored all our contacts, we all used to remember several phone numbers for our main family members and friends. Sometimes even some places (like work or school). Now we don't bother and most of us only remember our own number.

I called my girlfriend of six months yesterday, and she didn't answer, so I waited through the automated message to leave a voice mail. It struck me that I had no idea what her phone number is, not even the exchange. It takes me a second to remember my own number.