What Should I Do?

Several times this year I have mentioned finding old friends on Facebook, and how there is one particular friend, Casey, to whom I was especially close in high school and college, that I can't find.  She was such an interesting person back then, so smart and funny.  She was tall, with slender legs and arms, and my short little self was so envious of her effortless elegance.  She rooted for underdogs, liked to cook, was too shy to speak to any guy she thought was cute, and despised her curly hair (she wanted to have a sheet of silky hair, like a Veela, but her hair defied her).

To recap:  we kept in touch after I moved from Virginia to Florida for the first few years, then our long-distance phone calls and hand-written letters became farther and farther apart.  Communicating was so much harder in the dark days before the internet!  I haven't heard from her in more than 15 years, at this point.

I've asked other friends, and the same thing happened with them.  Casey moved away from our small town, and lost touch with all our old friends.  When people found me on Facebook, they'd message me asking about Casey, assuming I'd be most likely to still be in touch. 

Then I found, just by random searching on Facebook, Casey's sister Donna.  They were close to the same age, and we'd all been friends.  Thrilled, I friended her and sent her a message.  She accepted my friend request silently.  I messaged her again, and heard nothing.  I sent her an email to the address she listed, and heard more nothing.  I sent friend suggestions to people from high school we'd known.  Nothing came of that, none of them are friends with Donna - in fact, despite a fairly large number of our classmates on Facebook, I am the only one from back in the day that Donna is friends with. 

The one day out of the blue she posted a comment on one of my photos, saying she had some old photos she would post soon.  I was encouraged, and posted several old photos of her and Casey and I, tagged her to be sure she'd see them, and posted this on her wall:   Just uploaded some pics from college, several of you and several of Casey. Send me a message or an email, I'd like to catch up! :)

That was on April 11.  Since then, there's been no activity at all.  No response, no wall posts or status updates or anything.

So. I did a Google search for Donna's name and the city I see she now lives in, and found a phone number. I'm practically a stalker.

What should I do now?  Should I call her?  Should I take the Facebook silence as a big hint, and go away?  At this point I feel that the news about Casey might well be bad, and that bothers me tremendously.  Knowing what happened, even if it's bad news, would at least bring an end to my wondering.  But is that selfish, expecting Donna to relive whatever emotion is tied to thinking and talking about her sister?

Also, she didn't give me her phone number.  Would it be invasive to call her?  I found it because it is listed in the public phone directory, though, so anyone with access to a phone book can find it. 

Not ever finding out what happened to my old friend would be so sad.  And there is at least a TINY chance that Donna would be glad to hear from me. Maybe she just couldn't get the hang of Facebook so she stopped using it.  Maybe Casey is fine, and Donna would give me her phone number, and we'd renew our friendship, and everyone would be happy.

I could use some guidance, as I am completely conflicted about what to do.  Should I call Donna and ask about Casey, or should I let it go?  Please leave a comment and give me a little advice.  :)