I am having a bit of an issue on Facebook, please tell me what I should do?

[I<3MyBeatleBabes]

So yesterday, for example, was my Facebook friend Sandra's birthday. I know that she is a relative (my grandpa's cousin) but I don't actually KNOW her. We've never met or anything. Should I have written happy birthday on her timeline? I didn't, because I thought it might be weird to wish her happy birthday when I don't even know her. And then I have this other "friend" (who is actually a mutual friend who I added just to be nice. I don't actually know her) who posted yesterday about some very hard things she was going through in her life. I felt maybe I better comment on the status, but then decided not to, since we don't really know each other and I wouldn't know what to say. Then this morning she posts "It's interesting to see who has been posting on my statuses these past few days...and who hasn't." It made me feel kinda guilty. I mean, of course I feel bad about what she's going to. Whenever you hear a sad story in the news or whatever you feel bad for the people involved, right? But you usually don't say anything to them, because you don't know them personally. Basically what I'm asking is, how do you know what is and what's not appropriate to say to Facebook "friends" who you don't actually know very well?

[Fatal sniper]

Well First of all DON'T add people you dont know on facebook because u never know who they are in real life. And the same thing happened to me once too but I know the person just didnt talk to them alot but I never comment on their status or anything i just mind my own business I already have too much Bullsh*t to worry about