I'm gonna go with boring, stuffy, etiquette. Follow the addressee's lead. IOW, if you are responding to a forum post that only has a moniker, use the moniker until/if the person gives you his or her name' then use that. So I'd respond to the posts above, in sequential order:
John
J.K.
Dan
Duke
Bob
GS5556
Trelja
and finally
Newbee.
Just my $0.02.
John
J.K.
Dan
Duke
Bob
GS5556
Trelja
and finally
Newbee.
Just my $0.02.