Recently, Facebook introduced notification sounds.
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Like this one.
This is what popularity sounds like in 2013.
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But there's a hidden Easter egg within...
It's actually this chord: the Fmaj7.

The first note is F.

Then A.

Then C.

Then E.

Which is to say...

Consider our minds very gently blown.
The full story: Facebook's audio designer answers "How much research has gone into developing the Facebook ping sound?"
quora.com
It's also based on international ring tones — and the doorbell-like sound of someone calling "LASSIE!"