Brown commented on the post with a flushed face emoji.
Rihanna hasn't replied.
The pair dated between 2008-2009, before the rapper was arrested for physically assaulting his singer girlfriend - hitting, biting and choking her.
He was found guilty and sentenced to 180 days of prison and 5 years of probation.
Rihanna's fans were quick to condemn Brown for his comments on her post.
Many attacked him, calling him names like "loser" and warning him to "stay away."