When I backpacked around Europe a few weeks ago, I only responded to reviews on http://www.hostelworld.com/
. But that might be because I had actually booked the hostels through them. When I went around the UK in 2008, it was the exact same thing--only on hostelworld. Though, I usually only review if I either really loved it or really hated it. :P
I know that wasn't one of your options and it's not the exact same thing, but I just wanted to tell you about that one in case you might want to consider it. :) I've never reviewed anything on Yelp or trip advisor. I'm not a big fan of Yelp for hostels or hotels because it isn't directed specifically at them the way something like hostelworld is--it seems more directed towards food. I haven't spent much time on Trip Advisor, but I think it looks a lot better than Yelp for hotels/hostels. But what I like about hostelworld is that it breaks everything down--people get to vote on what they think of the character of the hostel/hotel, the staff, the location, cleanliness, etc. rather than just a 1-5 star overall rating.