I'll use people's real names when it doesn't cause confusion. e.g. when Seekeerz puts up the K M Links game threads, lots of people there greet her with "Good morning, Steff", so it's obvious who's being referred to.
Equally, when I'm specifically responding to one particular person's post (and it doesn't really matter whether others know who I mean or not), I might use their real name. For example, if Wolf63 reports a low score on a BBC Quiz thread, I might respond with "Better luck next week, Susan".
However if Wolf63 had, say, contributed an answer on a health matter (and particularly if there were lots of other answers there too), I'd try to avoid writing "As Susan has alredy said", because that would be confusing for the questioner if they didn't know who Susan was.
Anyway, my own real name is Agatha ;-)