At what point do you give up on a friendship?

I recently did, with a friend of many years, from highschool.
We would always at least wish each other a happy birthday, but this year he did not acknowledge my message, and I haven't received one from him for the last few, so it definitely seems one-sided. Even social outings, he never made the effort, so time to sever, me thinks. Especially when there are people in my life who want and deserve my time.