Nellie,
Boys are afraid of dealing with their emotions. Big time. Breakups are hard and the harder it is on the boy, the more he’s gonna act out (usually, through partying). He will go great lengths to prove to himself, the world and most of all you that he is ok (happy even) about the breakup and not sad at all! One could say he’s never felt better! So free! So light! But it’s all a front. From my experience, the harder the breakup, the harder they try to convince everyone how great they are.
Do not be surprised if you see him with another girl. Men/boys need someone to sponge off those emotions that are too hard to face. It’s the same as the partying; the more they are broken up, the faster they try to find someone else to distract themselves with.
Women do the opposite: we feel sad first and when we’re slightly better, we go out and try to live again. That boy’s gonna take much longer than you to get over the breakup because he chose to delay healing (dealing with the bad feelings, the loss, etc.).
So I know it’s hard to look at his happy pictures of partying but know that it is an act (no matter how convincing).
Focus on your own healing and stop looking at Facebook! Good luck