Chris Brown -- While Rihanna's Away ...

With Rihanna in New York City on her Diamonds World Tour, Chris Brown celebrated his birthday at Emerson nightclub in Hollywood ... with ex Karrueche Tran in attendance.

We're told Karrueche -- Chris' other on-again/off-again girlfriend -- was invited by Chris himself to the party (thrown by ZING Vodka and Black Pyramid) to celebrate Chris turning 24. Our sources say the two still talk often and hang out, and that she still cares about Chris very much.

Here's to hoping some combination of these crazy kids can make it work.