Most Helpful Guy
No. You should hangout with your ex, because you like him as a person and want to enjoy his company for the sake on enjoying him. Not using him to spite a guy you're not dating exclusively.
That's called being a user. Your ex probably won't mind, but in terms of what you "should do" is entirely based on your motivations.
Most Helpful Girl
It's 2am. If you're asking if you should have sex with your ex because the guy you're seeing is paying attention to another girl, I'm answering no. Don't think u can get back at a guy by hanging out so late at night with another one. However if you just mean "general hang out no hook up" then yeah you should get out there. Preferably in public looking good