Amicable exes, indeed!
"HAPPY BIRTHDAY ROBYN!" Brown, 23, tweeted early Monday -- the day the "We Found Love" singer turned 24.
Hours later Rihanna responded: "Thanks!"
Although Brown is currently dating aspiring model, Karrueche Tran, multiple sources confirm to Us Weekly that he and Rihanna have been secretly hooking up over the past year.
A rep for Brown insists to Us: "He and Rihanna are just friends."