I do a lot of very informal writing online, mostly in Subreddits related to my field, but also in comments on news articles at places like Wired and Ars. I find that you learn almost as much from the (non-insane) comments as you do from the articles themselves. This is probably the same reason I use/read Reddit.
I do have some more academic writing published on Academia.edu and SSRN, but I don't engage in discussion there.
Best of luck!
mercurialshark|12 years ago