John Shearer / WireImage 1 / 10
John Shearer / WireImage 1 / 10

Amicable exes, indeed!

Three years after Chris Brown infamously assaulted then-girlfriend Rihanna (whose real name is Robyn Fenty), the Grammy winner continues to fuel hookup rumors, publicly reaching out to her via Twitter.

PHOTOS: Rihanna and Chris' controversial romance

"HAPPY BIRTHDAY ROBYN!" Brown, 23, tweeted early Monday, -- the day the "We Found Love" singer turned 24.

Hours later Rihanna responded, "Thanks!"

PHOTOS: Rihanna's hair evolution

Although Brown is currently dating aspiring model Karrueche Tran, multiple sources tell Us Weekly that he and Rihanna have been secretly hooking up over the past year.

"Rihanna loves to live dangerously," says a pal of the Barbados beauty. "Talking to and hooking up with Chris is all part of that."

PHOTOS: Rihanna's edgy style

A rep for Brown insists to Us, "He and Rihanna are just friends."

MORE FROM WONDERWALL:

Rihanna's Fashion: Hit or Miss?

Vote on Rihanna's bangs

Rihanna and other celebrity catwalkers