AceShowbiz -

Ian Somerhalder and Nina Dobrev ended their relationship, Us Weekly reported. "The Vampire Diaries" co-stars parted ways after dating for more than three years. An insider told the site that the breakup wouldn't stop them from working together on the CW hit drama as they would "remain best friends, which is where the relationship started."

On the same day the split news was broken, Thursday, May 9, Somerhalder posted a cryptic message on Twitter. He shared to his 4 million followers, "A THOUGHT FOR TODAY: People change and forget to tell each other. -Lillian Hellman, playwright (1905-1984)."

Somerhalder and Dobrev, who always kept their romance private, went public with their relationship back in 2011. In an interview with Seventeen magazine in September 2012, Dobrev opened up about her relationship with the 34-year-old actor for the first time.

"I didn't want to be dating one of my costars -my goal on the show was to be professional," she said at that time. "But sometimes you can't help who you have a connection with, and you can only fight it for so long -which I did for a really, really long time."

Dobrev and Somerhalder will reprise their roles as Elena Gilbert and Damon Salvatore respectively when "The Vampire Diaries" returns for its fifth season next February. Dobrev will also star in Fox's movie "Let's Be Cops" alongside Damon Wayans Jr. and Jake Johnson.