Friends or Colleagues?

I store close to 700 telephone numbers in my phone. When I go out, I bump into people I know all the time. Many of them have got a working relationship with me. But just who are the friends and who are the colleagues? This is one way of looking at it:

• A friend is one whose number I will keep even after he has outlived his relevance to my work.
• A colleague is one whose number I will delete as long as he is no longer useful to me.

Sad, then, on occasions when someone whose number I have kept replies 'who is this?' when I send an sms to them. It means that I am no longer useful nor worthy as a friend.