Ooooohhhh; it's ON! After Liam's reported hookup with January Jones, Miley's not taking this lying down ... Ms. Cyrus ditched her engagement ring (we had the EXCLUSIVE photos yesterday) and now she's partying with a mystery man!

Miley's new boy picked up the singer last night and the two rode together to a friend's birthday party in North Hollywood. He arrived in his own car and then took the wheel in Miley's Porsche Cayenne, chauffeuring MC round-trip.

At pair spent two and a half hours at the shindig -- our photographers heard "Happy Birthday" being sung inside. We spotted her boy drinking a glass of wine and Miley enjoyed some too, we're told.

The two headed home fairly early - around 10pm.

So just WHAT is going on with Miley and Liam? Is their engagement officially OFF? Will Miley Tweet a rant about the media jumping to conclusions about her relationship? Will January Jones announce that Liam's the father of her child? So many questions ...