Answering this as a logical question would seem to include "lifnei ivar lo siten michshol" which also includes "blessing" a deaf person who would not hear you. When you say embarassing an "anonymous" person, does this mean for example causing embarrassment to someone by using his (or her) login name (such as my SabbaHillel reference). In this case, since I know that I am being referenced (even if no one else does) then it would indeed be lashon Hara and malbin pnei chaveiro. If you make up a name that no-one knows, then it is "cursing a deaf person".
In either case both are asur and the person causing the embarrassment has violated the commandment in the Torah. The Chafetz Chaim points out that unless there is a necessity (such as preventing a crime) one must not speak lashon hara (which implies that the statement is true). Of course if the statement is not true, the it is motzi shem ra which is forbidden as well.