What to do after receiving poor academic recommendation(s)
Your professor either wrote the good recommendation they promised, in which case they are probably as shocked as you are, and happy to discuss what might have happened, and what to do now; or they lied to your face, in which case I wouldn't worry if talking to them is "socially acceptable." I don't know what would be the best course of action if the second is the case, but my first step would be to talk to them in person - just don't assume beforehand that it's one or the other, to make the meeting productive.

As two comments pointed out, there's also a (small?) chance that the professor meant to write a good recommendation, but failed at it. If that's even remotely a possibility, as well as probably in the first case above, it might help if they contact friendly faculty (should there be some) at one of the universities applied to, to better understand where the bottleneck is (within what is possible, considering the confidentiality of the process). Given that a letter-writer usually has some interest in the student they write a letter for succeeding, I wouldn't dismiss the idea before bringing it up with them, if appropriate.