I have the same problem. I have opened a thread here, two in XDA and one in Microsoft Answers... I hope they fix it, or anybody comes up with a workaround at least...
Well yesterday i noticed there are issues for the first time.
Not WP to WP. But PC (facebook) to my WP. I get the first message or two from the one who writes to me. I answer and then I get no answer back at all. Only to find out that if I login to Facebook I have a message waiting for me there but it did not come to the phone. Nor does it if I continue the conversation on Facebook whereas before the messages always appeared on both my phone and facebook.