INTJs seem to have the longest attention span, with INTP second. I remember seeing walls of text on INTPcentral. This forum is more manageable, especially with Shai and Slant keeping us from taking ourselves too seriously.
The ENFP personality does not lend itself well to forum-style communities. I saw a facebook discussion comparing the distribution of types to the size of their respective facebook groups. ESFP, despite being a very common type, had the smallest group. INTP and INFJ, despite being rare, had some of the largest groups. I'd think that ENFP was closer to small than large.