"They did break up," a source tells PEOPLE of Jenner – the son of Olympian athlete Bruce Jenner and songwriter Linda Thompson – and his former Playboy Playmate girlfriend Nicole.
Nicole, 23, coyly addressed the split on Dec. 18, when she Tweeted, "Sometimes when you truly love someone you have to let them go...I never really understood that quote until now."
The following night she hosted a party at Chicago's Manor nightclub, without Jenner, 26, who was in Australia working. He left L.A. for Australia on Dec. 11 and returned on Monday. Just two days before he left for Australia, Jenner was still Nicole's number-one fan, re-posting her Tweet for fans to vote for her in an AskMen.com poll: "I'm up for the TOP 99 of 2010 pls go vote lots!" she Tweeted.
Last January, Jenner, told PEOPLE, "We're completely smitten [with] each other. I'm loving life." He added of Nicole, whom he had known for two years before they officially began dating, "The reason why I love this girl so much is because she knows me. She gets it. ... She knows my background."