I guess for me it just depends on the extent of the moaning.
If it's an occasional "Gah this heartburn is horrible" that's one thing. But going overboard and updating almost every hour about how horrible the pregnancy is, that's something entirely different.
Either way, I just roll my eyes, and hide the person's posts if they get too overboard. I know I moaned a bit with my first pregnancy, so I can't really begrudge others for doing the same thing.