Justin Bieber -- Impromptu Concert ... You Really Like Me!!! (VIDEO)

Justin Bieber brought the house down at 1OAK in WeHo Saturday night with a mini-concert.

Bieber took the stage and sang, among other things, "What Do You Mean?," and then rapped with Tyga.

It's interesting ... even Bieber seems a little surprised at his popularity among adults, asking "How many people thought you would like Justin Bieber?"

He's referring to himself in third person, so he's not that humble!