Looking on the internet deeply has found these results:

disappointed in you is the most popular phrase on the web.

disappointed by you 1803 results on the web	disappointed in you 1294,000,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
Sep 1, 2004 ... I agree with VenusEnvy, "I am disappointed by you" sounds awkward, but I think it is correct to say, "I was disappointed in your behavior / by ... Really great work, again and again, none of us has never been disappointed by you, wish you all the best and hopefully, you will keep editing for a long time, ... Jan 3, 2013 ... He proposes new, more accurate language for brides and grooms to employ at the altar: “I promise to be disappointed by you and you alone. Jan 30, 2016 ... Europe is disappointed by you, muslim loving, anti white, regressive traitor. STFU and GTFO you dumb cunt, you failed miserably.. Read more.	I am extremely disappointed in you, Karma. I was a bit disappointed in you. He is so disappointed in you... mindedness. I am disappointed in you, Mineko. I'm disappointed in you, Thane. Tara, I am very disappointed in you and your small I'm disappointed in you, Erica.

disappointed by you

1803 results on the web

disappointed in you

1294,000,000 results on the web

More popular!

Sep 1, 2004 ... I agree with VenusEnvy, "I am disappointed by you" sounds awkward, but I think it is correct to say, "I was disappointed in your behavior / by ...
Really great work, again and again, none of us has never been disappointed by you, wish you all the best and hopefully, you will keep editing for a long time, ...
Jan 3, 2013 ... He proposes new, more accurate language for brides and grooms to employ at the altar: “I promise to be disappointed by you and you alone.
Jan 30, 2016 ... Europe is disappointed by you, muslim loving, anti white, regressive traitor. STFU and GTFO you dumb cunt, you failed miserably.. Read more.