Yeah it's a typical facebook problem. I avoid it and stopped reading what people post because you get a really distorted idea about how happy everybody is and what great things they are doing all the time.
for me too this is a big problem. whenever I meet people they tell me good news for them and it gets me down and I feel even bad because I am not happy for them.
so I try switching the topic or sometimes I just say that what they are telling me makes me depressed and I want to talk about something else because it reminds me of what I lack.
sometimes people just keep talking after I said that and so I know I shouldn't meet them so often. but I think it's okay just to say it if you don't want to hear something if it doesn't come across rude.