A knock-knock joke has been cited in print since at least 1976:
 
Knock, knock.
Who’s there?
Ya.
Ya who?
What are you so happy about?
 
The joke became relevant to Yahoo.com in this version posted on the newsgroup alt.humor on January 30, 1998:
 
Knock Knock
Whose There
Ya
Ya who
Yahoo Internet life.
 
       
Wikipedia: Yahoo!
Yahoo! is a web services provider that is wholly owned by Verizon Communications through Oath Inc. and headquartered in Sunnyvale, California. The original Yahoo! company was founded by Jerry Yang and David Filo in January 1994 and was incorporated on March 2, 1995. Yahoo was one of the pioneers of the early Internet era in the 1990s. Marissa Mayer, a former Google executive, served as CEO and President of Yahoo until June 2017.
 
It was globally known for its Web portal, search engine Yahoo! Search, and related services, including Yahoo! Directory, Yahoo! Mail, Yahoo! News, Yahoo! Finance, Yahoo! Groups, Yahoo! Answers, advertising, online mapping, video sharing, fantasy sports, and its social media website. At its height it was one of the most popular sites in the United States
     
15 June 1976, St. Louis (MO) Post-Dispatch, “Jokes by You,” pg. 5D, col. 3:
Knock, knock.
Who’s there?
Ya.
Ya Who?
What are you so happy about?
Helen Pancella
City
     
Google Books
Make Me Laugh Again
By Sam Schultz
New York, NY: Penguin Group USA
1984
Pg. ?:
Knock, Knock. Who’s there?
Ya. Ya, who?
I didn’t know you’d be so glad to see me!
 
13 January 1985, Aberdeen (SD) American News, “Handful of city staff feel better after meet” by Cindy Uken, pg. 5A, col. 2:
“KNOCK, knock.”
“Who’s there?
“Ya.”
“Ya who?”
“Well, I’m glad everyone is having a good time.”
   
Google Groups: soc.singles
As sensitive as a pit bull
james.j.dutton
2/11/92
(...)
Knock Knock.
Who’s there.
Ya.
Ya who.
Nice to see you too.
 
13 June 1993, Chicago (IL) Tribune, “Spots,” comic sec., col. 1:
Knock, Knock. Who’s there? Ya. Ya who?
A. Glad to see you’re having a good time.
Ashley Myers, 7, Antioch
   
14 March 1997, Vancouver (BC) Sun, “Meet a Kids First Club Kid: Kurtis Rock,” pg. C9:
Favorite joke: Knock, knock
Who’s there?
Ya
Ya who (yahoo)
 
Google Groups: alt.humor
knock knock jokes
Pete Cowan
1/30/98
Knock Knock
Whose There
Ya
Ya who
Yahoo Internet life
   
Google Books
Nutty Knock-Knock Jokes for Kids
By Bob Phillips
Eugene, OR: Harvest House Publishers
2009
Pg. 108:
Knock, knock.
Who’s there?
Ya.
Ya who?
I didn’t know you were a cowboy!
 
Twitter
Annette Kerr‏
@annttkerr
@modernmami Knock knock -Who’s there? - Ya- Ya who? - Yahoo!!!  ;D #psknock
5:01 PM - 5 Apr 2009
 
Twitter
Saronti‏
@saronti
Knock, knock. Who’s there.  Ya. Ya who? Ride ‘em cowboy! #kidsJoke http://is.gd/vGLI
10:24 AM - 8 May 2009
 
Twitter
Travis Modisette‏
@tramod
“knock knock”...“who’s there?”...“ya”....“Ya who?”... ” .coommmmmm”
7:40 AM - 13 May 2009
 
Twitter
Antonio‏
@tonyabella
Knock knock! - Who’s there? Ya Ya who? …dot com. -_____________- http://tumblr.com/x5t75icjb
9:42 AM - 7 Mar 2010
 
Twitter
Carrie‏
@WhoIsCarrieHaze
knock….knock… who there???ya…..ya who??? Fuck Yahoo its Google Bitch!!!!!!
12:26 AM - 16 Mar 2010
 
Twitter
anom‏
@anom
Knock knock | Who’s there? | Ya | Ya Who? | No, Google…
12:35 PM - 16 May 2010
 
Twitter
1man1$elf‏
@1ofOneTae
”@_JustFollow: Knock knock Who’s there? Ya Ya who? I prefer Google” | haha
5:52 PM - 16 Jun 2011
 
Twitter
Edinburgh Festival Fringe‏
@edfringe
It’s #BadJokeFriday time again!
Knock knock.
Who’s there?
Ya.
Ya who?
Sorry, I prefer Google.
Think you can do worse? Share your puns, jokes and one liners on #BadJokeFriday
9:00 AM - 24 Nov 2017