Thanks for the question and thoughts. Shane and Rudy are right that the reason Facebook is required is to confirm the recommendation is genuine. Additionally, it makes it much tougher to game the recommendation system as a whole, which happens on other sites.
If you've ever looked through the reviews of some ratings or auction sites you can see that either the person being rated gets batches of reviews submitted within short time frames or many reviews made anonymously - sometimes by a competitors who uses the opportunity to drag your name through the mud. They do the former to make themselves to appear credible when in actuality they aren't and the latter to ruin your reputation. Furthermore, it hurts the truly great people from surfacing to the top - where they should be in the first place and really is the point.
The Facebook requirement ensures that every recommendation is given by a unique individual - with a name and photo attached so you know whose recommending you. As a result we're seeing some amazing reviews being written by users on Trulia.
The future of marketing yourself is not just having recommendations & reviews (testimonials) of past clients posted somewhere online - its taking that testimonial and continually broadcasting it to your client's friends. Requiring Facebook makes this possible by socializing the search for an agent. Trulia can help your client's friends find you and because they can see someone they know who has recommended you, they are 4 times more likely become your next client.