For posters, I think it's the charisma, uniqueness of posts, and accessibility of the person's ideas. People who provide a spectacle or food for thought (but as long as the food isn't too thick). Even if the person is inarticulate and aggressive with no friends, they can still be "popular" in a rough sense of the word. I could give better examples for INTPc, but this is MBTIc, so I'll make do.
Ivy - Very articulate, very intelligent, fun, and lots of SJ know-how with an N personality. It's not a common combination.
CaptainChick - Very outspoken, and charismatic towards a certain crowd. Very polar in her opinions.
Edahn - Outspoken, articulate, fun, engaging, and offers a lot of theories which are accessible and cause people to think at the same time.
I think it generally needs to be some sort of combination like this. People with only one of these things don't tend to be special enough to garner more attention than average.