Friend or Foe?

We often hear from people what makes a great friend. Most of the time we hear this in a form such as... "she/he's so thoughtful, like a true friend".

So what makes a friend, all of a sudden not become a friend?
How do you determine who is considered a friend and who is no longer considered a friend?

For example, I have a friend who I have been friends with since the summer of 2003. However in the summer of 2004 he moved away. We remained friends for awhile after his move, emailing each other back and forth every so often. I would also call him every year on or around his birthday to wish him a happy birthday, because that's what friends do.
For the last year or so, I have e-mailed him on occasion, mainly to let him know I was still alive and to see what he was up to. But he has never once e-mailed me back or even acknowledged my e-mails. I am starting to consider this friendship flatlined.

Am I right to be concerned about the lack of communication, or do I come off as being obsessed?
Should I continue to pursue an old friendship or should I just give up and live with the memories?His birthday just recently came and went...... I did not phone him this year.