Can I just say thank heaven I'm not the only one thinking this. It's really disheartening to slave over a story and put it out there for everyone to read and to get no feedback on it. I feel like I'm writing in a vacuum much of the time and I agree with you, I'm wondering why bother posting stories. I haven't been involved with fan fic as long as the original poster and I know that I have'nt reviewed as often as I could have in the past, however I'm trying to change that now.
I spend hours putting together a chapter that typically has around 4000 words, is it so much to ask that people who read it leave a small comment. I'm not even saying that I expect everyone who reads my stories to post glowing reviews about how they love every word I write, it's perfectly alright not to like it but tell me about in a constructive way that helps me understand what I'm doing wrong and how to correct it. I may not always make changes especially if it's more of a difference of opinion than me making a mistake but I'll always listen and respond.
I've made a personal resolution that I will review every story that I read, unless it's completely awful. If people are reading my stories and not reviewing then I have to assume that they consider the story awful and I won't waste their time or mine with further updates.