Ahh, a similar situation has happened to me.
One of my friends disappeared after a year of me knowing her. We had mutual friends and talked every day. She's been "missing" for two years; we have ABSOLUTELY no idea what happened to her. We've tried EVERYTHING: emails to all known email accounts, spamming the living heck out of her YouTube account, going on any websites that she somewhat knew of, and anything else.
After all this time, nobody has heard from her. She was always a bit... odd, and most think she just got into trouble with her parents because of her... oddness. She had this thing for plotting people's deaths. ;-;
Anyway, if any of my close friends from online went missing, I'd assume something happened to them, of course. If I considered them "close," they would at least have the decency to say goodbye to me. I'd be sad for sure because I'd miss them, confused as to why they're missing, angry as to the fact that I had NO IDEA why they were gone...etc. :D