We once pointed out that there's a difference between social introversion and the introversion of functions mbti talks about. What the latter means in that sense my colleagues above pointed out already and you can read a lot on the forums about entp and intp differences.
But to be a socially introverted or ivory-tower entp is no uncommon thing; if you think about how an entp is defined via Extroverted Intuition, it is in fact a thing that happens quite often and by some people it's even liked to be seen as insane in the membrane
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But as an entp you'll notice that your internal drive, which is most often defined by the things you are enthusiastic about will always bring you back into contact with the real world and eventually bring you into positions of responsibility where you can work with others on the projects that intrest you. This drive in an entp can have different dimensions, ranging from joining groups that care to make the world a better place and move into countries to go against the regional criminal establishment or becoming a gangster boss and forming a local thug squad that break noses of business partners they aint happy with; or a rather more introverted seeker of the new and intresting that delves in silence in front of his computer about fantastic movies, theories, ideas or watches brilliant movies and sees a deeper meaning in it and a big picture conveyed to him, which i.e. moral approach to things would change the real world for the better aswell.
One thing's for sure tho, entps are anything but common and in 9 out of 10 places you'll visit in your life, you'll be considered as uncommon, strange or even alien always giving you the feeling that you dont belong resulting in you thinking you're introverted.
To those people, in all circumstances, always keep a good "fuck you" in the backhand, cause if they would see themselves thru your eyes and figure how much they suck, they prolly never gonna find the concinsce to say a word again