I quite often just lurk around. Hell, it took me a year of lurking to actually make an account, and even then i didn't post often.
Nowadays i post more, though i quite often open a thread, read a bit, look at the "Reply to Thread" box where I'm supposed to write something, and stop. Sometimes i write a sentence or two, and then hold backspace, and leave. Don't know why.
Sometimes i do think "what's the point" as i don't really see anyone reading my posts often, especially in bigger threads. If a thread has over 5 pages and the topic isn't something close to me, the chances are I'll be leaving without a comment.
Still, my posts-per-day ratio is going up. Nowadays i post about 3 or 4 post per day. At least i think so...