Hey, you know all those tabloid reports about Katy Perry and Rihanna not being friends anymore because Katy doesn't approve of Ri's relationship with Chris Brown?  Well, they're not true.  How do we know?  Because Katy went out of her way to send Ri a lovely -- and profane -- tweet wishing her a happy 25th birthday.

Rihanna turned 25 on Wednesday, and Katy marked the occasion by tweeting, "HAPPIEST 25TH BIRTHDAY to my baby girl @rihanna! I wish I could b there drinking Mai-Tais & telling haters to eat a d*ck w/u!"

For her part, Rihanna tweeted a black-and-white photo of herself and Chris Brown from several years ago, and captioned it, "#throwbackRiRi my surprise 20th birthday party!! Im an old b**h now! Thank God for His gifts and His endless mercies! #25 #MilestoneLife."