I know this question is better posed to the 'powers that be' at Trulia but as a realtor, I'll give you my two cents anyway. :) Some of the sites load directly from our MLS system, some do but only once a week or so, and some don't at all. I manually key in my listings here, and some other sites that I won't mention here. Because they allow us to reference your MLS#, then they can pull the detail from that site, but it's not an automatic feed.
To make sure you're exposed to as many sites as possible takes time because there's so many that appear and disappear. I always tell my clients that the most accurate and timely is our own MLS system, rather than the feeder systems. I will manually input the data to as many that my client and I identify but many times there's some inaccuracies in 'value' on those other systems too. There's just no policing for accuracy of information on sites that either 1. are not directly fed from the MLS or 2. are not keyed in manually by the agent/seller.
Sorry if this was more than you wanted. I look forward to hearing what Trulia says directly?