I saw this questions on Quora,

"How much research has gone into developing the Facebook ping sound?"

I thought it was an interesting question because I have never really given it much thought. Then the actual sound designer answered the question with a little science.

"I designed most of the audio currently in the product today. There are a few major components that make up the audio identity. First, is the base chord which is an Fmaj7 chord. For you music nerds out there, yes, these notes actually spell out "FACE". There are a few reasons I went with this chord."

You can read the entire answer on Quora and an article about the ping on Fast Company Design.