The issue you describe is so common, psychologists have a name for it: "ambiguous loss." It refers to people who are mentally present but physically absent (as in the case of your missing online friend) or physically present but mentally absent (as in the case of someone who has a mental illness).

I've written elsewhere about a book on ambiguous loss. And after reading this, I may see if I can dredge up that post and maybe rework it for Medium because that kind of loss affects even more than the dozens who have responded in your comments. Thanks for the inspiration :).