No one knows what they really are.
That being said, I recently met a girl who said her first impression of me was "Intelligent but unsure of yourself." She may have been either an ESTJ or ENTJ. I think this is the impression I often give off when I am reserved or with strangers in real life.
I haven't asked any of my older friends what they think of me, simply because asking "What do you think of me?" could be perceived as a pleading urge. If I were to have a friend ask me such a thing, it would seem like a difficult and possibly drawn out assessment that couldn't really be compartmentalized into anything brief. It would also seem like they would be longing for either a 'deeper' personal connection or maybe a soul-search. I suppose it's a valid question, but asking it would disclose too much.
If I were to ask such a thing, it would skirt around the issue. For instance, "How outgoing am I?" or something of that nature. It would be easier to dig into specifics.
Now that I think about it, most people who meet me tend to ask me questions about general topics, such as history, science, or whatever. I think this probably indicates that they think I am a receptacle for knowledge or something, but it could simply mean that they don't know the answers.
Online, however, people probably see someone who is more tangential. I have been characterized for the same aspects in real life, on top of "mysterious", "attention whore", "direction-less", "multi-faceted to the point of multiple personalities", "hard to figure out" and "imaginative". I hold all of these to be true, and to be different aspects of the same person, which is me. Oh yeah, and the newest ones - "sicko" and "asexual".
I also think that the longer people know you, the more space they give you to be something other than their first impressions. This may sometimes lead to long-time friends just refraining from either praising you or diatribe-ing you because you just simply "are" to them and nothing can change that.
In addition, my ENTP friend thinks I'm, on a scale of 1-10, a 6 on the introversion/extraversion spectrum.