There are several places in the virtual world to memorialize these friends. Memorials seem to be very popular, in fact. There is even a special Rainbow Gardens for beloved pets ... pets whose only contribution to the virtual world was barking in the background or chewing on the cables (stop that, Collage!).
The loss we feel for these friends, even ones we didn't know all that well or hadn't marked as "Friends" officially, is magnified by all the other losses in our lives. It reminds us of our bigger losses, our saddest times, and we can weep for those again as well, hopefully washing away that pain bit by bit but never entirely losing sight of it. The sadness we feel can also remind us of the happiness we shared. The dancing, the jokes, the mere sharing of time and virtual space.
So don't try to tell us our virtual friends aren't real.