There are some people who sell their home without an agent. Although it is a very small percentage, it can be done if you are very knowledgeable about what you are doing. And if you are one of those people I want to congratulate you for taking that on and wish you the very best.
However, although there are different levels of advertising on the Trulia site, all of us have paid for this advertising in one way or another. Whether we choose to use the advanced offerings of Trulia or we are allowed to list via another source, we pay for this service one way or the other. Some large companies offer this as an incentive to agents to join their firm and it is automatically downloaded to this site. Some of us pay for the privilege of listing on MLS and we then are allowed to promote those MLS listings to other internet sites.
This is a very abbreviated answer to your question, but I hope it helps to clear things up a bit for you.
Trulia doesn't accept FSBO listings at this time, however please keep in mind that if you choose to work with either a full-service or flat-fee broker or agent in the future, he/she can submit your listing to Trulia via the link below.
You can list your home For Sale By Owner (FSBO). However, you cannot have your FSBO listing appear on Trulia. Trulia feeds from the MLS which only contains listings by Realtors. So, if you want your home on Trulia, you will need to hire a Realtor.