I think we have all been there. Try not to focus your attention so much on what they are doing, how much they have logged onto facebook etc. The more you think they are ignoring you, the more you will focus on it and ignore the other great relationships you have. (Including the relationship with yourself)
It is sort of like a Murphy's law: The amount of traffic is always inverse to the amount of time you have to reach your destination.
In other words, sometimes it is just appearances brought to us by our minds.
On the other hand though, sometimes people are trying to "start over" and make a clean break. Or maybe they feel really guilty about something and don't know how to tell you, so they just ignore you. This can become a vicious cycle, where the more they ignore you the more guilty they feel, and the less they want to contact you, etc...
I think an interested thread to contrast to your question is the one on ending friendships:
Ending friendships
Sometimes people just change, and maybe they can't relate anymore. People are never the same, and you should not blame yourself.
Also have a sense of abundance about the relationships you do have. If you come across as needy, people are less apt to want to hang out with you again. Think if you ever had a friend who seemed needy. So the best thing to do now is focus on the good relationships you have, and I bet you that this person will contact you. Don't be angry when they do, as anger will only hurt the both of you. Instead, talk about the good times you had, and all the things you have done in your life.
Become a person who is happy alone or with people, who doesn't really need anyone, and I assure you people will start to contact you all the time. I am not saying to be unfriendly, in fact, be more friendly, and confident about who you are and the relationships you have. I know it isn't easy, I struggle with it everyday, but it can be done, and I think one feels better for it. If you think about it, you focus on this friend because they ignore you, and they might only be ignoring you because you focus so much on them. Switch these two things, and everything should be fine. Good luck!