There have been threads where, over the course of a few weeks, members have racked up thousands upon thousands of posts. If you don't really care about the quality of the content it actually isn't all that hard. See, for example:
http://www.flyertalk.com/forum/misc....sted&t=1148414
http://www.flyertalk.com/forum/misc....sted&t=1126028
http://www.flyertalk.com/forum/misc....sted&t=1001243
http://www.flyertalk.com/forum/misc....osted&t=884884

For a while I was averaging over 50 posts/day not in that sort of thread. Yeah, that was more time than I probably should've been spending here but I enjoyed it. That said, I did spend my first couple years not posting very much at all so no way I'm near the fastest to get there.

ETA: Between July 22, 2010 and December 15, 2010 I had 5,444 posts.