Disappointed Beliebers were scratching their heads over why JB didn't hop up onstage to join his old mentor -- which was widely expected, but didn't happen. Instead, Justin was in the stands with Hailey moments before the Halftime show kicked off, and he was chillin'.

Loads of fans channeled their emotions through comedy online ... with hilarious memes trickling in on X as it became clear Bieber wasn't joining. Some mocked how they were patiently waiting for JB to make his grand entrance, others poked fun at him vibing in the crowds with everyone else, and some even threatened violence over the result. Jokingly!

JB wasn't the only one catching some heat over sitting out of what would've been one of his most memorable performances ever -- some raging commentary was directed at Usher himself.

Many pointed out he brought out so many people like Alicia Keys, H.E.R., Lil Jon, Ludacris and will.i.am -- but couldn't nab the one person everyone actually wanted: Bieber. Of course, the decision might've been out of his hands ... and we know Usher tried.

We broke the story ... Usher had been in direct talks with Justin lately over sharing the stage together -- so it looks like he did his best on his end to execute the moment that would've rightly broken the internet ... but it looks like the Biebs just wasn't into this.

As to why he didn't go ahead with it ... it's anyone's guess really, lots of possible reasons.

It goes without saying, Usher was a key figure responsible for catapulting Bieber to monumental fame when he broke out on the scene as a fresh-faced 13-year-old. That's why a lot of people thought he'd return the favor by performing at halftime, but alas ... nada.