Knock knock. Who's there? Citation needed.

Citation needed who? This joke requires a source to be funny.

Knock knock. Who's there? Flake.

Flake who? Flake out, I'm tired of waiting!

Knock knock. Who's there? Dwayne.

Dwayne who? Dwayne the bathtub, I'm drowning!

Knock knock. Who's there? A placeholder for a name.

A placeholder for a name who? Exactly. It could be anyone, couldn't it?