When is it time to throw in the towel on a friendship?

So I have this friend, she used to be my best friend. She used to be up for anything; drinks, a party, shopping, a movie. We talked and shared a lot throughout 3 plus years of friendship. You would call or text her and she would always be there. Now, not so much. She has fallen off the face of the earth. I know people have lives, full of work and relationships. But friendship to me is just as important.

“The causes of modern social problems, from divorce to homelessness and obesity, are often thought to be based in areas such as poverty, stress or unhappiness. But researchers suggest we are overlooking something crucial: friendship. It would appear that our society is ignoring its importance. The philosopher Aristotle said, “In poverty and other misfortunes of life, true friends are a sure refuge. They keep the young out of mischief; they comfort and aid the old in their weakness, and they incite those in the prime of life to noble deeds.” Friendships are vital for wellbeing, but they take time to develop and can’t be artificially created.” -http://psychcentral.com

I used to call and text her weekly to attempt to make plans, even for an hour just to catch up. Usually she didn’t even get back to me, but when she did she always had an excuse. We have hung out maybe 6 times in the last 8 month and talked on the phone maybe once. I don’t even consider this a friendship. It’s not like she’s moved away, not like she is in jail or unable to communicate in some other way. I’ve thought about it a lot lately and I’m leaving the ball in her court. I will no longer attempt to contact her or make plans. We will see if she tries to contact me…

What is the importance of friendship to you?