If you enjoyed working with your agent, a simple thank you note pointing out what made him/her special, should be sent and addressed to the broker owner of the realty office, and a copy sent directly to the agent. If you were not satisfied with your agent, you can discuss all issues with the broker owner and the agent as well.
As a local agent, I would encourage you to write a positive note about your agent if that is your intent. If you have an issue with the agent then I would certainly take it up with the agents broker and let them address the issues that you bring up. I see no value in trashing an agent in this form if that is what you intend to do.
My hope is that you received great service, honest consultation, an objective opinion of the process, ( if you purchased- of the purchase price) and much more that the majority of agents in Atlanta stive to provide to buyers and sellers alike.
On this site you could write your review in the form of a blog and allow people to comment. If you are looking for a specific site other than Trulia that accepts reviews, i would just suggest to google real estate agent reviews and check out the many different sites offered.
Your blog here though would be seen by many different buyers, sellers, agents and people just looking.