If you post it, they will reply.
Seriously, even if you echo what others have said in the thread - Everyone's post matters.
Post some original questions, thoughts, observations or art/writing work...If this doesn't work for you, write in your journal? People definitely read those.
When I first started posting on TFP, I felt as insignificant as ever...until someone replied. If large threads are intimidating, post in smaller ones or create your own. I also realized that posting in threads that I had interest in (not just what was popular) and could relate to: TiltedParanoia, TiltedLiterature, TiltedPortraits and, of course, my Journal, proved to keep my interest and gave me plenty to type in my replies. Remain within the rules (respectful and tactful), but speak your mind.