Today I was chatting with three of our visiting graduate students from the PRC. Thinking that I was being au courant, I mentioned the expression DUI4XIANG4 對象 ("boy/girl friend" < "target; object"); I knew very well that no one would say something so creepy and out-of-date as NAN2PENG2YOU and NÜ3PENG2YOU. But all three of them (two women and one man) simultaneously laughed and said, "That's so old-fashioned, Professor Mair!" So I asked them, "What do you say now?" I was amazed when they told me, "We just say 'BF' and 'GF'." Of course, I knew right away that they meant "boy friend" and "girl friend," but I thought such usages were confined to short text messaging, chat rooms, bulletin boards, and so forth, where they are indeed extremely widespread. What struck me is that "BF" and "GF" are part of their spoken Chinese vocabulary as well.
TA1 SHI4 WO3DE BF 他是我的bf ("He's my boy friend") is a perfectly good Mandarin sentence. I suppose that one could refer to this as a kind of code-switching, but I suspect that BF and similar expressions function as assimilated Chinese terms. Acronymic loan words? I'm really not sure what to call them, but they certainly are prevalent in the language as spoken and written today.