Despite the 'Im New Here' declaration I have been on PPRuNe for a number of years and retired my previous 'handle' for a few reasons.
To answer your questions Fuji, I read some threads post by post and scan the others. I rarely add to the discussion as having only been in aviation a few years feel I am not properly qualified or experienced to have anything worthwhile to say. But thats not to say I don't spend a lot of time (too much 'her indoors' would argue!) reading, considering and learning from many of the discussions.
I gleam a lot of information from threads which are particularly relevant to me, be they current topics or some very old threads. Only this morning I was reading through a thread from 2005 to try answer a question I had.
Generally, as a rule of thumb, I find only the first page or two (three on occasion) to actually be useful. Anything after that has normally sidetracked the original post and has entered a new discussion around a new topic, or more often than not a willy waving contest. PPRuNe seems victim to this more than most other forums I visit. I have often wondered the reason for this.
I find if a topic comes along which I can comment on it is crucial that I find it early on and make my post. If I don't see it for two or three days I don't comment as although I do have input to share it would be entirely out of context on Page 5.
My rambles,
4B