Most people are very likely to use a persona in real life as well, to stick to their ideal, be loved/admired or fit in. I have a very good example of that with one my closest friend who has been typed by other acquaintances several times as ISTJ, ISTJ, ISFP, INTP, INTJ and finally INFJ.
She tends to play a role aroung people to fit in and be liked - it works more or less. It is very obvious when you learn to observe people and patterns in their behaviours but it does not really tell which type is their real type, some people are very complex and good at faking everything.
(I am personally more likely to be myself on the Internet than I am in real life because I have severe anxiety issues, lack of social skills and am a bit obsessive about things.)
Anyway, while it's fun to type people, I really think we should encourage them to type themselves if they want to rather than believing we can guess everything about them. It's a ridiculous belief that one can read perfectly into other's people souls or anything like that : we can guess through observation but some things will remain a mystery and we may be wrong.