A while back I tried signing up for some Chinese BBSes to practice my written mandarin (I believe I tried mop.com and 163) and I found it utterly impossible to figure out what to do. I was expecting something similar to reddit, HN or traditional phpBB sites and it was nothing like that at all. Do you know some sites that might be easier to figure out?
I would suggest http://www.douban.com/
Much cleaner user interface (just forget about mop.com or 163.com, those are ancient sites...)
Better content, people discuss about books, movies, music and much more(you can try "groups", there're so many with a wide range of topics)
And www.guokr.cn -- Geek topics goes here. :-D
Sorry, as far as I know, we don't have web service like reddit or HN. Like @v-yadli suggests,douban is good site, most of its user is student and young artist(文艺青年).If you
want to write long articles, you can try www.diandian.com, a light weight blogger like tumblr.
Chinese forums live in the bad old days of the early 2000s. If you're afraid of the plunge, you can come post in the Chinese-language-only section of Chinese-Forums.com; we're a friendly bunch.