The problem always comes when 'your' (as in the generic) comfort zone is wider than the other persons.
For instance, I have known Ast, the thread originator, since the first day he arrived her on elliquiy.
I am very fond of him and have on occasion, sent the odd email to him, just to see how he was doing (though I missed his birthday this year...*sobs*), but I know very little about him, simply because I 'feel' he doesn't feel the need to tell me.
(Your ears burning yet Ast?....*grins*)
I try to treat people online, as I would face to face.
Some you find are very open and you get to know them quickly, others may take a long time and some you will never be more than a passing acquaintance to....